I'm sure hoping the release of D7 doesn't take the thunder away from UC for 6x.
There are so many unfinished D7 releases, beta, alpha,etc. Some of them will be months away for finish release.
I don't have a single production D7 site up now, and I've given D7 a good try 4 times expecting better things.
No doubt D7 will get there, and some of the features are very attractive.
At this time in the real world we continue to need the best UC possible, hopefully with additional improvements.
Drupal 6 is still the only way to go, if a competent shopping cart is a requrement.
The D7 is really being pushed out there, and no doubt in many applications it is just fine.
In my particular situations D7 just doesn't fit at this time.
I'm not trying to discourage use of D7 for sure. I'm just saying again... if a good working UC is important stay with D6 for now