of course you can double knit in the round.. (in some ways, its easier--you keep track of pattern on front, (never turn!) and the inside takes care of its self!
you can use any fair isle pattern--
decreaseing is bit tricky (for say the crown of hat)
you need to use a cable needle (and position 2 knits together (to K2tog) and 2 purls together (to P2tog)
more complex decreases (double decreases ) are possible.. but... well, more complex!
if you are on ravelry, there is a double knitting group--and many members of the group have "object patterns" (my blog has an 8 part tutorial (link above is to one page!) and i have many stitch patterns... (vs object patterns) my stitch patterns are over sized swatches and/or potholders..
The Big Book of Knitting has a some good info on double knitting too..