Assuming a battery backed controller, the data should be flushed out to disk and you would be fine. Power supply failure isn't really any different from shutting of the machine unless there was some massive surge.
That being said, raid isn't backup, you must have backup.
Also, if you can afford it, two power supplies on two different power sources ("A/B" power) is always good. I don't have any actual evidence, but from my experience PS is the second most likely thing to fail after hard drives (excluding maybe fans). Anyone know if that is true? I know I have more HDs than power supplies, but this has been the case for me.