The supply is often used as an AC "ground", or return path.
As you noted, "good" bypass cap's are required. Now just what
"good" means, there's the kicker! There's been quite a bit
of "rule of thumb" written on the subject, and lately there
have been some good analytical attempts.
Another thing to watch for is resonances - especially between
the ground and supply planes if they are a significant portion
of a wavelength at your highest frequency in operation. These
can be a real pain to fix.
-- --------------------------------------------------------------- Mark Randol, RF Measurements Engineer | Motorola SPS, Inc. (602)413-8052 Voice | M/S EL379 (602)413-4150 FAX | 2100 E. Elliot Road [email protected] | Tempe, AZ 85284 ---------------------------------------------------------------