There was a recent post w/ a similar question & a lot of replies:

http://forums.gardenweb.com/discussions/2809928/help-me-try-again-with-butterfly-weed?n=16

I planted a whole bunch of seeds in the ground a few yrs ago. Only one came up but got huge. Last year it spread about 10 ft away and made 2 babies..probably via seed since there's a sidewalk in between them and the mother plant. I finally saw one Monarch butterfly last year so it took them a yr to find the plant. I never saw any caterpillars though. I hope I get some this year.

In my experience, milkweed is difficult to transplant, even from a pot. They seem to be very temperamental about having their roots messed with, which is odd since it is sometimes considered a hard plant to get rid of. With the plants from the big box stores, I would just sink the pot into the ground and let the plant reseed itself. Growing from seed, I keep mine in a shaded location every time I pot them up and I try to get to them before they become any kind of root-bound so that I don't disturb their roots while repotting. Right now I have about a dozen each of prairie and whorled milkweeds that I recently potted up from cells to 4" pots. I lost one of those to transplant shock. The next pot-up will be straight into their places in the garden where I will be sure to water daily and also screen (but not block out completely) from much of the sunlight until I'm sure they've taken. A better way to go might be to get the 4" peat pots and sow the seeds directly into those and then drop the pot into place when it's of good size. Once you get a plant or two to go to seed, they will reseed themselves quite happily around your yard and seem to do a far better job than we do.

OK, thanks - maybe I'll try that. I only have one or two small plants hanging on from last year, and I'd really love to get some established. I've heard they are pretty sensitive to transplanting, so maybe I'll have better luck doing it your way.