The 3.5 DMG errata for polymorph states that hit points do not change even if the new form has a different CON though some would use the PHB beats DMG for spells argument which could be a valid point
The equivalent of polymorph in the MM, which is alternate form which follows pretty much all other rules of polymorph also states that hit points remain the same. Sometimes this is a penalty (werecreature) whereas sometimes an advantage (dragon)
Having a hit point increase from polymorph is barely an advantage anyway as one dispel magic later you could be dead. I would certainly never rely on it. Wildshape, not being dispellable is another matter