[Update Apr 2010: As far as I can tell, Brute have unfortunately discontinued this product, so it is probably going to be tough to find]
The possibility of cauliflower ear has hovered in the back of my mind since I started BJJ, as I'd prefer to keep my ears as they are. I haven't had any major problems with my ears up until now, except for a brief period of general soreness on my right ear, which seems to have faded.
Nevertheless, that prompted me to buy some protection, for the same reason I use a gum shield: I'd much rather pre-empt injury rather than wait to get hurt before acting. For this particular injury risk, wrestlers are the most experienced, because cauliflower ear is common in that sport. I've even heard that in some countries it is viewed as a desirable goal. I for one don't wish to replace my ears with grotesquely twisted lumps, which meant I needed to invest in some head gear.
There are numerous brands of head gear (the term varies: I've also seen it referred to as a headguard, earguards, head protection etc), so as I'm not in the know when it comes to wrestling, I had a trawl through BJJ internet forums to see what people recommended. The name that kept cropping up was the Brute Shockwave (e.g., this EFN thread): as I could find it fairly cheap on eBay, I plumped for that.
The Brute Shockwave is lightweight, comfortable and at least so far, durable. I can still hear ok with the headgear on, as the material has small slits by the ears. I also didn't find it made any noticeable difference when somebody was trying to choke you, as I'd previously thought it might (except that people trying to crush your head when they can't finish the triangle is a bit less painful).
You do need to be careful if you have any cuts or scabs on your upper neck (such as if you were a bit careless shaving that morning), because that will chafe against the strap: I've got some kind of spot thing on my neck at the moment, so haven't worn my ear guard for the last few lessons. Men (and some women too, depending on how puberty went for you: contrary to popular belief, it isn't necessarily just men who have a protruding prominentia laryngea) will need to watch their larynx, as if you wear it too tight, that will also rub against the strap. Apart from that, it doesn't take long to get used to wearing headgear.
There are adjustable straps on the front and back of the head, as well as on the neck. I'd suggest experimenting to get an ideal fit, as you need a balance between choking yourself out with the strap and having it spin around your head due to being too loose. Leaving about two fingers between the strap and my neck seems to work for me, but then I have bony fingers. ;)
After a while, depending how much you train, sweat accumulates throughout the head guard and starts to stink. I had been told that it's even more pungent after washing, but putting it in at 30 degrees and then hanging it up to dry seems to work fine. I wouldn't advise spin-drying, though I haven't tried it myself, and you should also be careful of the velcro: try not to leave any exposed when you put it in the wash.
This is a fairly common product, so its available all over the place. If they're in stock, you can buy one here for $16.99 (or if you prefer a UK supplier, they're available here, but for £30). Personally, I bought mine from this guy's store on eBay. That worked out at around £20 (which included shipping to the UK, so reasonable), although as a small supplier I'd assume stock varies. More eBay options below: