Almost all my hdr bracketed shots are done handheld. With the 7D's fast fps, 3 bracketed shots at once and good light, it's not very difficult. I also shoot 5 or 7 bracketed images handheld with my Mk II. Like shooting a rifle, good breathing technique and locking in how the camera is held really helps. In low light conditions or when I want to do more than 3 brackets using my 7D, then I use a tripod.
The newest versions of Photomatix and CS5 do an excellent job of aligning the bracketed shots for tonemapping.
The longest exposure I've done hand held on a single image is one second. Keep practicing and you'll do fine.
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