ETA: Here is how I made the cylindrical part of the Camelbak sock:
2.5 mm circular needle (magic-loop used).
Top down, starting with a long-tail cast-on over two needles held together.
48 stitches, joined in the round (24 per needle).
R1: * K2 P2 *
R2: P * K2 P2 * K2 P
I could not work out how to knit a circular bottom to the cylinder and even considered crocheting the base because I know how to crochet a circle. Instead I decided to try to work it out for myself and, after a failed attempt that was to tight too quickly (knitting from the edge into the centre) and then making a similar sleeve for hubby's sigg water-bottle, I finally worked out a pattern for the base that worked (on his first and then I fixed / re-knitted the base of mine).
Here is what I came up with, written out as a pattern:
Circular base of a 48 stitch cylinder.
Knitted in the round. In my case using magic-loop.
Row 1: K48
Row 2: K48
Row 3: * K6 K2tog *
Row 4: K42
Row 5: * K5 K2tog *
Row 6: K36
Row 7: * K4 K2tog *
Row 8: K30
Row 9: * K3 K2tog *
Row 10: K24
Row 11: * K2 K2tog *
Row 12: K18
Row 13: * K1 K2tog *
Row 14: K12
Row 15: * K2tog *
Row 16: K6
Row 17: * K2tog *
Break yarn and thread it through the remaining three stitches. Weave in ends.
This is the first knitting pattern that I have designed and written out. I am sure that it has been designed/defined before as a method but I could not find anything and so worked it out for myself!
Some photos in this post and more over at flickr (click through).