I asked myself the same question... wait or buy now. I was so eager to have the Android experience I bought the Captivate upon release.
Now I'm so glad I did.
Frankly I find it's all too easy to follow the pack so to speak & become hyper focused on " the next great version" like Froyo for example. I'm here to say that I decided I would use Eclair 2.1 to learn Android, and then think about upcoming versions.
That's the best decision I've made. First, upon learning eclair and doing just a few things to optimize it (without rooting) I'm very pleased. I have zero lag, battery life greater than my iPhone4, and one sweet, fast & stable smartphone. And it still gets better. A former BlackBerry enthusiast, for it's stellar keyboard & email champion, I've never been fond of the touchscreen keyboard ... till now. SWYPE as implemented on the Captivate is just plain impressive. I'm at least 30% faster than I was on the BB, a statement I never expected to make, and it's really fun to use.
So, the bottom line for me has been nothing but good news, a great experience, and my burning desire for "the latest & greatest has diminished due to the excellent experience I'm enjoying with my current setup. It's also important to note I'm quite picky, so the point being if I'm happy with the present Captivate, that's a pretty good endorsement.
Go for it, you'll be glad you did...