Installing a different PORTVERSION based on OSVERSION?

[Darren Pilgrim]

I'm working on a revision to a port where the current version doesn't 
work on 8.0-R; however, an older version does.  Rather than roll the 
entire port back to an older version, I'd like to have the newer version 
available to 7.x and older and the older version available to 8.x and 
newer.  Is it appropriate to install different versions of a port based 
on ${OSVERSION}?  If so, do I just set the necessary variables after 
including bsd.port.pre.mk?