I've known how to do this forever, but never got around to actually doing it until last week. I have to say, I'm thrilled with the results and it has to be my most favorite sewing repair ever.

In order for the jeans to not look stupid and obviously hemmed, you want to preserve the original hem. You'll need a sewing machine with an adjustable needle position and a zipper foot to accomplish this.

Basically what you want to do is to fold the jeans up to make a cuff, and then you'll put your zipper foot up against the original hem, move your needle over so that it will sew right up against the stitching of the original hem and sew. When you're finished, you'll flip the cuff back down so that the flap of fabric you've taken up goes up inside the pant leg. You should press it flat with an iron and if you want to you can hand stitch the flap up inside, but I didn't bother with this step.

The trickiest part is getting the length right. If you know the inseam you want, you'll fold the cuff up and measure from the crotch to the stitching of the original hem - not the bottom of the fold. Your inseam will then be about 1/4" longer than that once you sew and flip it back. It took me some trial and error, so you should either pin it well and try it before sewing, or sew one leg and try it before doing the other. I had to rip my stitching out a few times until I got it right, but I wasn't doing it based on measurements either because I couldn't figure out where I was supposed to measure to until I tried it a few times - spatial relations aren't my strong suit.

I did two pairs of jeans this way - one fairly heavy all cotton pair and a lighter weight cotton/spandex pair. The lighter weight ones look better, but they both came out fine. Much better than the awful "hemmed" look jeans get if you just cut them and shorten from the bottom.