Both of your current lenses are consumer models, and I wouldn’t consider them a reason to stick with Canon or Nikon. Moreover, the Canon dRebel line has a 1.6 focal length multiplier, and all Nikon dSLR cameras have a 1.5 multiplier, so your 28mm wide angle effectively becomes 45mm on a Canon or 42mm on a Nikon digital body.
But I WOULD get a Canon or Nikon. Plenty of professional wedding photographers use a D70 or dRebel as a their backup or even as their primary body, so you’d be in good company. These models are good value for money, they take great pictures, and with both brands you’ll have access to a fantastic upgrade path for bodies, lenses, and accessories. This last bit is where Pentax loses out, despite their new K10D.
I personally always thought the D70 was a tad better than the 350D, despite having 6 mega-pixels vs. 8 and the 400D is clearly better than both. However, I always suggest to upgrade lenses first, and then the body. Even six megapixels is plenty for an 8×10 inch print.
Nikon makes a very nice 18-70mm lens for around $310 retail (effectively 27-105mm). With the D70 and a decent memory card, that would keep the total price for Nikon under $1000. I don’t think Canon has a lens that can compete. One step up, and Tamron has a great 17-50 constant f/2.8 for $450 (for Canon and Nikon.) These options seem much more tempting than the added mega-pixels of the 400D. To me, anyway.
Canon and Nikon also both have great 17-55mm f/2.8 zooms (I use the Nikon version with a D200), even better 70-200 f/2.8 zooms, great 85mm portrait lenses, good flash systems, etc, etc. For semi-pro work, I’d invest in all of that before spending extra on the body.

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