Looks like it's a quick way to do multi-color work & Tunisian crochet - there's a really good tutorial here at crochetme.com. If you like video tutorials instead, here's one (fast forward about 30 seconds in to the start):
Also, here's a tutorial on using one of these double-ended critters: Basic Cro-Hook Tutorial. There's also something called Portugese Knitting... which is actually done with two tiny crochet hooks!!
How cool is that??
Anita (tygger428) has a double-ended afghan she's working on, which gives a good look at the hook actually running from one end of its project to the other here [Ravelry link, sorry]. Also on Ravelry, the Crochet Liberation Front Headquarters is running a discussion on the mysterious Circular Crochet Hook, and what can be done with same.
Best thing I've found made using one? Becky's absolutely amazing Hogwart's House Shields Afghan:
I mean WOW. Almost makes me want to make one! And there are lots of patterns that can use this little double-ended circular wonder. Yahoo! Answers has the best links for patterns, here, but I haven't even gone out on Ravelry to look yet, and I bet there are some there. (Runs off to check... I may have to investigate getting one of these Circular Crochet Hooks for myself!