I have the XSi with the EF-s 18-55mm IS lens. My focusing ring is sort of wobbley. I compared the EF-S 18-55mm non IS (XTi) lens at the store, since they had no XSi's on display and the focusing ring also felt wobbley.
I hope this is normal and not just a coincidence that mine and the one at the store are defective.

