Tubular cast on: in the round
With a waste yarn in contrasting color, cast-on half the number of stitches you need using the chained cast-on. If your pattern calls for an odd number of stitches, add one to that number and then divide by two.
Switch to your main yarn. Purl one row.
If you're using double pointed needles, at this point distribute stitches evenly (as possible) between four needles.
Join in the round, and purl three more rows. The wrong side will be facing outward.
Purl one stitch.
Now you're going to insert the right-hand needle into the purl bump from the first row. It's intermixed with the waste yarn. This is why using contrasting yarn colors is key.
Insert RH needle from top to bottom (front of the stitch to the back):
Place the stitch on the LH needle from bottom to top (back of the stitch to the front). Knit the stitch. That's two stitches tubularly cast-on! Repeat (purl one, pick up 'purl bump', knit 'purl bump') to the last stitch. If you wanted an odd number of stitches, purl the last stitch and you're done. If you wanted an even number of stitches, purl the last stitch and also knit the last purl bump. It will be a selvage edge, so it may be pulled fairly tight, and you'll have to work to get your needle in there. You should get something along these lines:
Do a few rows of K1, P1 ribbing, then you can start to carefully take out the waste yarn. If you undo the last stitch cast-on with the waste yarn, you should be able to pull the rest out by tugging on the end (which is what makes the chained cast-on a sweet provisional cast-on):
Voila! Tubular cast-on in the round.
Marina also inquired about the tubular cast-on with k2p2 ribbing. My instinct says that you would proceed normally with the exception of purling 2 and knitting 2 purl bumps instead of 1 and 1. I'm not sure if that would work out, so it's an investigation for another day....