Hi Graham
Keith is right - this exactly what I do [most of the time] with xara - I draw the line work by hand and use xara to color and put together the final image.
If you want to vectorise hand sketches as part of the artwork [as opposed to just using them as an on screen reference for tracing etc] then its important to get a high contrast scan. Dense Ink [black or red] works best, pencil can be a pain.
Gnurf's tutorial is good - xhris has also done one on bitmap tracing
I was saying 12 months ago I'd do a run through of the way I do it, and I've not yet got round to it :o - might just get it done this time round.
If you would like to post up an example of a scan I can certainly say how I would do it. It works best with a high resolution scan at normal scanner sizes [A4/A3] - 300dpi is what I use - so it might be an idea to post it up zipped at that resolution, otherwise it might be a tad big on the screen :D