Yes, you send the response to the person/people who invited you, wherever they are, irrespective of where the event will occur. Use the address they used for a return address.

If you really want to be totally formal and correct, you follow the wording of the invitation. Miss Manners even says to center the lines! You said the invitation is "worded formally." So your reply might look like this (imagine the lines centered):

Mr. and Mrs. William Doe [or however you style yourself]
accept with pleasure
the kind invitation of Mr. and Mrs. Smith
for
Sunday, August 21, 2005
at 6:00 p.m.

(For some reason, if you are declining, you not only change "accept with pleasure" to "decline with regret," you also insert "very" before "kind." Don't ask me why.)

If you can't stand to do it this way, with the centered lines and all (I did it only once, to a fancy schmancy invitation, sent from Italy, to a wedding in Africa -- I was declining! -- but I felt very self-conscious and strange, frankly), I don't see why you couldn't just write a simple note thanking them for the invitation and accepting -- you don't have to put in other information or chattiness, like you would in a thank you note:

Dear Mr. and Mrs. Smith,
Thank you for your lovely invitation. Mr. Doe [or "William"] and I are delighted to accept.
Sincerely,
Jamie Doe

If you think this sounds too terse, you could add something like "and look forward to joining you" or "and look forward to celebrating with you" or something at the end of the last sentence.

I agree with you that as you don't know them, and as they have sent you a very "correct" invitation (formal wording, no reply card), a casual reply ("Sounds like a blast! We can't wait!!") isn't what you want.

Both look just fine to me - I think I'll use the latter though, just so it doesn't sound *too* stuffy, ya know? ;-)The first one sounds almost uptight (though I know it's correct), and since I don't know them, I have no idea if they'd take a very formal reply as "sarcastic" or not, so better to stay on the safe side of things with something like the latter, I think.

Thanks a bunch! :-)

That's what I'd do, too, and for exactly the same reason.

Have fun at the wedding.

While the RSVP cards are not required, ettiquette-wise, it is correct to have printed in the bottom right hand (IIRC) corner of the invitation something like:
RSVP/The favour of a reply is requested
123 Four Lane, Fivetown, State, 78910
31 February 2005

which indicates where replies should be addressed and the date by which they should be received.

Your solution sounds like the right one- though you might include the date: "We look forward to joining you on the 21st of August" : this is a confirmation to the hosts that you have correctly interpreted the invitation (which is the reasoning behind the formal wording) :-)

I'd just write, "we were pleased to received your invitation and wanted you to know that we both will attend your wedding on July 26 at 4pm, and the reception that follows."

I'd mention both events so they know for sure to count you for both (where I live, it's not uncommon for people to attend one and not the other--usually they skip the ceremony itself)

Talley Sue, really? Wow, talk about regional differences. I think that that would be considered rude here, as if the guests were saying, "I don't really care about your wedding, but I'm not going to turn down a free meal." Occasionally I have seen people attend the ceremony but not the reception, though, usually because they are infirm or in mourning (e.g., at our wedding, a 90-year-old cousin of my dad's had died a week or two before. Her sons and their wives attended our ceremony, but not the reception, which included dancing).

Both in Cleveland, where I grew up, and in Boston, where I now live, it's common for people to skip the ceremony and come only to the reception. Often, though, this is when the ceremony is in the morning with a several-hours break before the reception.

As a child, I always thought this was just awful, and used to make the argument to my parents that if they were going to attend only one event, it should be the wedding ceremony.

Oh, I see. I forgot that in some communities, the reception doesn't immediately follow the ceremony. In our community, it always does -- I have yet to see an exception.

Are you sure this was a bona fide invitatiaon and not just an announcement?