no risks beyond bank foreclosure, and they are FDIC insured. you can remove your money at any time, though it takes a couple days to do so. the rates are variable, but they have been on their way up, not down, and you can always remove your money if you don't like the rate. really, i don't know how you could get any safer, since it is pretty much impossible to lose money if you keep less than the FDIC maximum in there.
edit, oh yeah, you generally can't take your money out for the first 10 days, but that's all. and you can always withdraw your interest, there's no waiting period or anything like that.
they are taxable, though. keep that in mind.