I haven't played it, but from what I've read and seen...
Yes, it matters which side you swing on: backhands and forehands are all part of the game!
Wii Tennis is complex enough to track the angle your racquet hit the ball at, and can calculate whether you sliced the ball or not. It can also determine how strong your hit was so that a weaker hit won't travel as far.
I don't know if it can actually detect you trying to lob the ball though...
I actually had similar "depth" questions regarding Wii Golf...
Rather, I think that Wii Sports is a simple, pick-up and play demo-esque experience that will get non-gamers hooked and give them a taste of what lies out there for them. If you liked Wii Golf, then you'll start looking to buy Super Swing Golf, or Tiger Woods. If you liked Wii Tennis, then you'll really be looking forward to Virtua Tennis Wii or whatever. If you liked Wii Boxing... surely there's a third party out there working on a boxing Wii game?
~Carmine M. Red
Kairon@aol.com