OK, I'm NOT an experienced photographer but I just went through about the exact same decision process that the original poster went through. FWIW I have a few observations here but keep in mind they are from someone just getting started.
Despite the advice from a couple people not to I went ahead and got the Canon 50d over a 40d and 5d. The 40d is a better deal and a couple hundred less expensive but I saw a couple of features with the 50d that I thought would be worth a couple hundred extra. The rear screen has much higher resolution and is better for checking your shots than the older screens were. The kids I was photographing would pose and then run over to see thier picture - having the better screen was cool.
It's probably stupid but I wanted a camera that had the same or better number of megapixels than my point and shoots. I have a G9 that does 12.7 so it seemed a step back going with a 10.1mp 40d.
I like the way you can fine focus the 50d using your computer.
I wanted the same processor in case I decide later to upgrade to a 5d MKII.
The 5d didn't have a self cleaning sensor, the 50d does.
I'm sure that the 5d would probably take better pictures but I was just a little leery of a used camera - the person I talked to in the store was.
The pop-up flash on the 50d is pretty much as has been mentioned here: mostly useless with the L lenses. They are big enough that they cast a shadow under the flash. My next purchase will probably be a better flash.
I tried a Tamron 28-75 f2.8 lens and didn't get that great of pictures out of it. The faster speed was good but the 50d wanted to shoot either too high an ISO or too low a shutter speed. You kinda have to watch both.
So far the best shots I got were with the 24-105L f/4, but they were outside shots. I still have more experimenting to do with the f/2.8 speed lens. If you were going to start out with one lens I'd go with the 24-105l f/4 IS to start with.
-- just some camera thoughts just in case they might help.
A couple comments about Sitka Deer and Stetson. Even though I've had a couple "extended disagreements" with Sitka Deer about LOP and whatever he has still been extremely helpful about wood finishes, adjusting triggers etc.. Even though we don't always agree I've probably learned more from Sitka than anyone else here on the forum.
Stetson has been VERY helpful to me about the camera's even though I bought the body he recommended against. On the forum and with PM's he's gone out of his way to help me out.
In short I know that both Sitka and Stetson are good helpful posters here, I hope that even though they disagree they can work it out. Both have with me............................DJ
