In all honesty, the 1D is a bargain. It does lose out in AF speed/accuracy to the Mark II/n version, but the differences are quite small and the average person would never notice the differences in all honesty they are that close. Noise? Yes, the 1D is noisier, but up to ISO 400 I don't think you'll see that much difference between the original 1D and the Mark II variants. And if you do see noise, use something like Noise Ninja or Neat Image to fix it. Amount of MP? 4mp is fine for most people. I've had several 8" x 12" prints done of my macro work and the quality is outstanding. I've had the photolab going "wow, what camera took this!" Cropping might be an issue, depending on how much you're cropping and if you nailed exposure, but it's certainly do-able to an extent. The higher MP counts of the Mark II variants really do help here.
A major issue is the lack of a zoom on the LCD screen, and battery performance is very average from my experience. The other issue is long exposures, the amplifiers for the panasonic made CCD sensor are very noisy and over a minute and they'll be obvious...this really should have been rectified by Canon imho.
Do you really need those super large print sizes, and do you really consistently shoot at higher ISOs all the time?
A good advantage of the 1D is the pixel size does allow a better resolution, especially when stopping down (as I do in my macro photography), this may or may not be important to you.
The 1D is an absolute bargain, I bought mine for AU $2k 3 months ago and don't regret it. When you think about it, a 2nd hand 1v film camera is going for around $1.8k in Australia, 2nd hand...so going digital is a few hundred dollars more.
As to pulling out the sensor from an XT and putting it in a 1D - forget it.
Dave