Well, I just discovered today in a PBW game that ramming ships can't ram planets... darn, and I had some level 6 warheads on those things too!
(Junkyard game if you were wondering about the tech level)
The weird thing about it is the ships move into position to ram... and then they just sit there, like the planet is blocking them from reaching the one square of the planet they are trying to ram! Didn't we have a similar trouble with seekers a while back?
Good thing the planet was unarmed though!
(It SAID all but seekers on the target list, and drone warheads work, so I'd ASSUME that ships could ram planets as well
)