Having upgraded from a 350D to a 30D this year, I have to say it's a fairly decent step up. There's a noticeable improvement in AF, especially AI Servo, as well as the ability to shoot at 5fps. (As a motorsport photographer, it was like a breath of fresh air compared to the rather limiting XT). Image quality remains about the same, although the "keeper rate" is higher with the 30D.
The larger size, the Quick Control dial in the back and the "standard" LCD location on top of the body are all welcome upgrades over the more compact layout on the XT.
With the frenzy over the 40D, this is a pretty good time to snap up a 30D on the retail or used market.