I have noticed this two-hour rule in some of HP's discount fares also. I think it depends on the mood/diligence of the agent, how full the flights are, and whether you can justify the delay (appeal to any humanity that the agent MAY possess).

I've never tried to standby for a later flight, so I don't have any anecdotal evidence here. I don't recommend calling if you see this restriction in your fare rules (see Travelocity or ITN), b/c it might get noted in your PNR that "PAX advised no standby after two hours of original flight departure". It's alot easier to convince the gate agent you were told the opposite if there are no notes in the PNR!