Eli,
It is extremely common that redwood will weep its tannic acid and stain whatever is in its path.

Muriatic acid is used all the time by masons to clean masonry.
In the correct diluted amount it will not harm your concrete.
It's toxic in uninformed hands. So read up on it before you use it.

Take the water to the hazardous waste facility ? You got to be joking. If that was the case I'd be there at least 3 days a week.
No, just adequately dilute the stuff or hire your masonry contractor to do it for you.

Your contractor bears no responsibility for this naturally occurring problem.
There would only be some sort of liability if you had stipulated in your contract that the redwood would be of kiln dried quality and that it was stain blocked sealed.
It is highly doubtful , due to the expense, that you had kiln dried redwood spec'ed and installed on your project, but check your contract and your set of plans just incase.

You however should get to know your specific materials and how to care for them.

For more in depth information about this very common problem go to the website of the Redwood Association _ http://www.calredwood.org/about/about.htm

Thanks for the info. I'm curious about one thing--you mentioned that I should get to know my specific materials and how to care for them, which to me implied that I had neglected to do something I should have. Do you have some recommendations for me? I asked the contractor about treating or painting the wood--he said we could just let it go gray like most others do. I also asked about sealing the concrete and was told that would make it to shiny and slippery. What is it that you think I should have been doing, so that we can take the proper preventive action once we get this thing cleaned up?

Just make sure your contractor knows how to use the acid and that he's willing to take responsibility for the outcome. I'm now dealing with a long sidewalk with ugly spots all over it because someone attempted to clean spots with the muriatic acid.

Hopefully, in your case, it easily cleans up with the correct dilution.

I would be extremely annoyed if my contractor essentially designed a deck and cover that was destined to do this. I would not consider it a "buyer beware" issue only. Now, if you had picked the 2 plans and materials and then just got a bid to have it installed, then you would be responsible for knowing how the materials would interact.

That's not a legal opinion--it is more of a "reasonable man" idea. So, I would not give up just yet on some kind of recourse or restitution, if indeed your contractor devised this disaster scenario.

Maybe this is the document DD was referring to?

Here is a link that might be useful: pdf about extractive something or other

Who specified the materials seems to be a very relevant issue - as is the communication process with the contractor. I agree with d-d that one needs to understand the materials you are building with - either by doing your own research in advance of the project or/and discussing them with the designer and builder. If I'm having something built with materials I'm not already familiar with, I make sure I learn as much about them as I can. And if I've specified materials a contractor is not familiar with, I make sure he learns how to work with them - or find another contractor! Last year I ended up introducing a contractor to white cedar shingle panels (a common product elsewhere but relatively new to this area.) So I think everyone involved in a project has an obligation to undertand the materials - the degree of knowlege required by each party will differ and good communication between all parties helps ensure that problems like yours arise rarely. I think contractors generally feel little need to 'educate' the client about materials, especially if the materials were specified for them - so that's why I said who specified the materials is relevant. But I'm a 'hands on' type participant in projects so I have different expectations that you may have had.

I asked the contractor about treating or painting the wood--he said we could just let it go gray like most others do.
You can let it go naturally gray if that is the look you want or there are other types of finishes to choose from. Read the link that karinl provided. It has all the information you need to know about how to choose the right sealer, paint, stain , oil or varnish for your redwood.
The key in applying the various different finishes is knowing when to apply them in regards to the moisture content of your wood. Moisture content is what is assisting the migration of the tannins to the outer surface of the wood and creating the stain dripping.
If it makes you feel any better, the tannin dripping will eventually stop as the wood naturally dries out.

I also asked about sealing the concrete and was told that would make it to shiny and slippery. What is it that you think I should have been doing, so that we can take the proper preventive action once we get this thing cleaned up?
Me thinks your contractor doesn't know sheet about concrete finishes.
Look up penetrating sealer such as a 501 inpregnator non shine or check out Prosoco's product line ( we use this company a lot ) I'm a big fan of their color enhancing sealer - doesn't shine, and can take a beating ( My favorite test was coating half a piece of bluestone with the product, then flipping a greasy hamburger on the stone and pouring a glass of red wine on it - NO Staining at all on the sealed side.)

A penetrating non shiny sealer will not make your concrete surface slippery.

I suggest you go down to your local high end masonry store and talk to the guys behind the counter. They'll give you the real scoop.

PS. Application of sealer on the concrete is also linked to moisture content.
You don't want to seal the concrete too soon after a pour. Let the moister cure out first to avoid mildew entrapment.