I tried Xsi in a store and I own xti (I tried xti and xt in a store too at the same time just to be sure). Xsi focused very fast. Xti was a little bit slower and xt had difficulty to focus. I put the same kit lens on all of them, just to be sure. Well it wasn't very extensive test, not a lab test too, but that was all I could do without buying it. Xsi has DIGIC III Image Processor and xti has DIGIC II Image Processor. So that is probably the reason.
In real life I sometimes get focus problems when I press shutter button and picture is not taken because camera can't focus. I know that there is some situations when automatic focus doesn't work, but I was getting problems in simple compositions.
I never tried Pro Canon bodies, so I cannot comment. I think the reason journalist uses Canon and not Nikon, is MORE pro bodies for Canon and better than Nikon, everything is more available and L lenses.
However my description as a photographer is quite different. I don't see myself spending $2500+ on a body, plus $1000+ on L lenses. I am not that good photographer and photography is sooooo far for being my bread and butter. I would like to improve my skills, but I don't see how more expensive body and lens will do that. I might get better pictures optically and they will have more megapixels, so what? Any SLR lens will be better than point and shoot. Bigger sensor will improve pictures too.
So 28-300 mm is too big, too heavy and to expensive for me.
55-250 is too much telephoto to make it walk around lens.
28-200 is good, but it would be better if it had IS.
So at this point 28-135 is better choice for me.
Lens creep... Well many consumer grade zoom lens will creep just because of the way they are constructed. Lens extends a lot and it just become too heavy.
Third party lenses? Might be that is the way to go?
The processor has nothing to do with autofocus speed.
Some Canon pro bodies cost under $2,500. For example a D5 with a full-size sensor body costs under $2,200.
If you read the XT manual, you will notice that the camera does not take the photo not because it can't focus, but because something in the way you are taking the photo does not match. You will see a sort of warning on the viewfinder as the green LED's flash on/off, except for in a couple of modes where you can override the camera's automatic selections by the camera.
Some L primes cost under $1,000 each. However, Sigma, Tamron, and Tokina make outstanding lenses for Canon (or Nikon, or Pentax, or Olympus). Most primes are fast, and lighter than those with IS or zooms.
Take a look at Canon and non-Canon lenses. You will notice several highly regarded Canon lenses under $1,000:
http://photography-on-the.net/forum/showthread.php?t=141406