4. Stitching - using a panoramic head
There are a few of these on the market, all of them allow for proper alignment of the shot images, by rotating the lens around it's nodal point. You have to calibrate for each lens you intend to use, but it only takes a few minutes to do.
I use the Nodal Ninja 5, which offers solid support but is correspondingly big and bulky, and you really need a levelling base on the tripod for ease of use. I generally put the camera into portrait (vertical) mode, and shoot 6 or 7 overlapping frames, depending on the lens.
This setup gives me around a 108 megapixel 3:1 image when using the 24 megapixel Nikon D3x, but I often have to crop this a little, as it's quite difficult to visualise the finished result. Still, plenty of resolution to play with.
There's a couple of options on the market for automated, motorised panoramic heads, onto which you mount the camera, programme in what you want in terms of angle-of-view, and let the electronics do the 'hard' work. Trouble is, I don't feel that the actual picture-taking is the hard work bit. Great idea I suppose if you need to repeatedly shoot the same angle of view, or need a 360 degree view for VR purposes. Gigapan sell a model suitable for use with SLRs. the Epic Pro, which is available in the UK from Red Door VR (they also sell Nodal Ninja products).
5. Stitching - using a shift lens
By mounting your camera rigidly on a tripod and taking a sequence of 3 images with a shift lens, 1 with no shift, 1 with maximum left shift and 1 with maximum right shift, you'll get either a 2:1 image when using full frame, of 3:1 if using DX format.
For my purposes, DX works best, and offers 32 megapixels when used with the Nikon D7000, enough for a print up to 30" wide at 300ppi. A 24 megapixel DX SLR would take this to nearer 40".
The advantages of this solution are that it is relatively portable, and also it's possible to use a tilt movement with Nikon's PC-E lenses, to give great front-to-back sharpness. Also, the images are nicely aligned with big overlaps so your stitching software has an easy job.
Unfortunately, tilt/shift lenses are quite expensive (although you could try the older Nikon AI shift lenses, but you need to watch compatibility as you can damage your camera depending on the model) and metering can be tricky. Also as with any stitching technique, it's difficult to pre-visualise the final result (although I find it a bit better in this regard than using a panoramic head).
(The above applies to the Nikon PC-E lenses, which offer +/-11mm of shift, but I believe Canon's equivalents are very similar)