Yes, you can use your laptop to play content on your television. Just buy a DVI to HDMI adaptor and a HDMI cable and connect it to your screen. For audio, you'll need either a mini-plug to RCA cable (for stereo sound), or better yet an optical cable (Toslink) with a mini-plug on one end. Toslink will give you 5.1 surround sound if you connect it to your A/V receiver.
You can use the built-in Front Row app to play your content, or you can install Plex, which is free. I have no experience with Plex, but there are lots of threads around here about it.
The main difference between using a computer like your MBP and an ATV is the tradeoff between power and ease-of-use. With your MBP, you will be able to play 1080p HD content, whereas the ATV only does 720p. The ATV is dead-simple to use, though, and the interface is better than Front Row's. That's one of the reasons people who buy Mac minis for home theater uses go with Plex.
As for your last question, no, there's no way to connect your laptop to you tv wirelessly. That would rock though!