I have had my xti now for about 2 years and still am happy with it. Even though i do love it, i would agree with anubis and get the 30d. one of my friends has the 30d and i asked if i could use it for a sporting event, and it just felt so much better than the xti. i was using a 70-200 f/2.8L IS, and this is a huge lens. on the xti it feels like very unstable and felt like the lens would rip off the mount on the camera. while on the 30d it felt very secure. not saying that the xti or xsi is a bad camera, i personaly would rather have the feel of a 30d or something similar.
as for the lenses, i hate the kit 18-55 on the xti. as for the new 18-55 kit on the xsi, i havent had a chance to test it out, but my hopes arnt very high for that lens. but the 28-135 f/3.5-5.6 IS is a better lens. i also had this on my xti and was a great deal better than the 18-55 kit. image quality was alot shaper while the IS was a great bonus, but the new 18-55 kit does have IS. the only downside to the 28-135 that i saw was that i wanted a wider lens. so i ended up getting the 16-35. but yeah kit lens on the 30d or any of those is worth it.
im not sure what the price is on these cameras now, but i think the 30d is pretty cheap now since the 50d is out. i just wished someone told me to get the 30d before when i was looking at cameras. good luck buying.