As I've had my machine for less than two months, thinking about a backup wasn't... uhm... something I considered. Not because I didn't want one, 'cause I'm pretty paranoid on the subject of backups. However, first, with the complex apnea, I have a Bipap Auto SV, and those beasties are not cheap, even at CPAP.COM's great prices. Second, they haven't been out long enough to have a bunch of them hitting the reseller's market. And third, if I didn't get one identical to my primary machine, I wouldn't have any idea what to get!
But having both a backup machine and backup power are high on my list of acquisitions before the 2009 hurricane season! I emailed the Battery Geek folks, who make the BG-222, and they said that within a couple of months, they will have a solar recharger for their batteries on the market.
http://www.batterygeek.net/