Nikon D3000- great follow up from the Nikon D40. Problems with it is a very dated CCD sensor that lags behind in high ISOs and it's not compatible with many lens(which should be a problem for you), but it has the best AF system of any entry level and has been reviewed very well.
Canon XS- This is probably the Toyota Camry or Honda Accord of the bunch, it's not outstanding in any area but performs very well in almost every category, for an entry level.
Sony A230- I'll just put it this way, it's already been discontinued and was considered, by many, a flop.
Sony A330- has the same problems, mostly design of the body, that the A230 has and has received low marks in reviews.
Olympus E-620- Has the best kit lens of any manufacturer, great for a light DSLR, very good all around, but it's a 4/3rds sensor, which means you don't get that shallow depth of field on that you'd get on the others(means how much it blurs the background). It's great if you are looking for a very small DSLR.
Canon XSI- These won't be around much longer with the new T2i coming out. The XSI has all the photography features that the XS doesn't have, better auto focus system, and faster shooting. The XS is designed more for the casual photographer and the XSI is designed for the more serious type but on a budget or wanting a lighter DSLR.
The one camera that isn't on the list is the one that I think is the best bang for the buck and best performer in this class. It's the Pentax K-x. It's a plethora of features in an entry class model and it's image quality is very good.
Here's the conclusion to the review of the Pentax K-x.
http://www.dpreview.com/reviews/pentaxkx/page29.asp