So far, I have not had any need for IS on my Nifty-Fifty. However, any upgrades are ALWAYS welcome, granted they work the way they were intended to do so. The only problems I have with this is;
1. 50mm f/1.8 IS, sounds like a replacement for the Nifty-Fifty, NOT the 50mm f/1.4, and
2. I agree with most of you on here, Canon will more than likely bump up the price significantly.
Now it is true that the Nifty-Fifty has too few aperture blades-which I don't mind as this has never been a problem for bokeh for me. If the 50mm f/1.8 IS included 7 or 8 blades, an upgrade in construction quality and the IS, then I wouldn't mind dishing out a reasonable amount, however, again, Canon would want $600 and not just the $300 - $400 you can currently purchase the 50mm f/1.4 for.
Those are my 2 cents.