The Scubapro G2 has this built in. From the manual:...
I've been looking through most of the Scubapro ones over the past few hours and they all seem to have some sort of display for altitude classes, even the more basic aladin models, so that's reassuring. I realise that it may not be something I really need to worry about straight away, but my drive up to 600m is quick... in 10-15 minutes. I live on the side of a volcano. So, it was something that concerned me. I feel a lot better about it now. I have 5 mins in the rib back to shore, 10 mins getting all the kit off the boat plus another 30 mins having a shower and changing before the 10 min walk back to the car, so I should be ok. I'm still a bit overwhelmed with the choice of computers out there, but at least I know I can find one that has what I want.