If you think you will be staying with crops for a while, stick with the 17-55, I promise you will not regret...
Last year when I decided to sell my 17-55 (upgraded to APSC-H), do you know what's my feeling?
I feel like I'm selling my boy away...
No conflict between EF and EF-S if you are using on crops.
EF-S doesn't work on full frame though, and that's probably the main reason why people doesn't choose 17-55.
The IS on the 17-55 helps quite a bit in low light environment.
When do you think you are going to own a 5D or 5DII?
One thing I don't like Canon full frame body mostly because it has a poor focusing system (compare to the price).
Make sure you know what you are shooting.
For landscape, portriat or non-moving objects, full frame is terrific.
However, for sports or concerts, IMO, it's quite crappy.