At the store the day I bought the Series 3 watch, yes, the Apple guy who was working with me did have a difficult time getting the stainless steel band off my old SS series 0 watch, but part of that may be that I had never changed bands since the day I got that watch two and a half years ago. I've kept the older watch and have put a sports band on it just for fun and if I eventually buy any more bands will use the Series 0 as the one which gets the swapped bands, leaving the black SS link band on the newer watch. Anyway, just mentioning that because I can see where there MIGHT be a problem connecting and removing a stainless steel band from an aluminum watch, if there is meant to be a snugger fit or something with the heavier band. I agree with Relentless Power that if you do go this route, be careful when inserting and removing the stainless steel band!
Next time I'm in the Apple store, if I remember, I'll ask one of the sales associates just out of curiosity what their thoughts are on using a stainless steel band with an aluminum watch.....