My own understanding of this is that Venus does not have plate tectonics
because it does not have a well defined crust. A lot is still unknown about
Venus, but it appears the surface is too hot for a brittle crust like Earth's.
I've never heard of the idea of tidal forces having much to do with plate
tectonics since seafloor spreading was discovered in the early 1960s (or
so?). Wegener's original theory of continental drift posited tidal forces as
the cause of drift. Modern (post-1960) theories of plate tectonics -- e.g.,
plate mosaic theory, which appears to be the current conventional wisdom
-- posit convection in the mantle as the proximate cause of plate movement.
The Earth's moon appears to not have plates because it does not possess
a the same kind of highly convective mantle as Earth. Ditto for Mars.
Cheers!
Daniel Ust
http://www.oasismag.com/Issues/9802/story-ust.html
http://www.mcs.net/~tshell/ust/homepage.htm
http://www.geocities.com/Athens/Acropolis/8422/ust1.htm
http://freeradical.co.nz/
http://www.teleport.com/~jaheriot/posthum.htm