yea, it should be a drop in replacement with ubuntu able to detect and use it out of the box.
No proprietary drivers, no firmware. No issues with sleep and wake, jumping from ap to ap or what not.
The only caveat with the ath5k driver is that it's new. It's still being very actively developed and as such, some newer chipsets dont have full features. You'll be able to do just about everything you'd expect to be able to do, but for instance, i have a desktop card that maxes at 18mbit/sec instead of 54Mbit putting me on a 1.2-1.4MB/sec bandwidth restriction. txpower auto-regulation is to blame as different chipsets have slightly different ways of dealing with it.