This mod is actually quite safe. I sold my kit lens to a modder who mods the lens for people that allows it to be used on a 10D. So far he has modded 4 lenses and all no problems..
i overheard at TCW last week when i was there, dat one of the guys there provide such a service. but, u need to come up with the lens. anyway, once modded, the lens can ONLY work for 10D. it won't work for other canon DSLRs.
the explanation he gave was due to the mirror flipping up. since 10D and 300D are virtually the same in design, the mirror will not hit the back elements of the EF-s lens. however, this cannot be the same for other DSLRs like D60.
anyway, if u managed to pull it off, it get a quite usable wide-angle lens for ur 10D at a fraction of the costs of getting a WA lens.
well, keep an eye for a EF-s lens in buy/sell? then u can either DIY or bring it to TCW to do it for you at a fee. a used EF-s lens usually goes for $80-$100.
The price of a Sigma 18-50mm is around $180...reasonable enough.
Anyway, just a quick clarification, it can be used with all EF-mount DSLRs with the 1.6x crop. Using it on a FF (think 1Ds or Kodak DCS Pro SLR/c) or a 1.3x crop (think 1DMkII) will cause vignetting. If it was a FF lens I'll would have gotten it for my SLR already...