@Sonic
Yep, especially if you are painting it, plywood is a cheap solution. Before I built my first laminate guitar I searched through a quite a few sheets of normal ply and was surprised to see so many sheets that were warped. Marine ply on the other hand seemed a lot more consistent. But marine ply is way more expensive, but for me its only one piece of 200x300. I can get a few guitars out of one sheet.

@Jim
It's all about warping. Pine has a bad rap for that. I've built a few guitars now with pine/ply/pine and they all seem to be holding up well.

rob