I wanted to reply to an archive messaged and question that was answered in the Answers section. Obviously, I couldn't! Anyway, after some searching around I finally found a forum where the folks were able to extract pretty much everything from the main.pak file. Here's the forum: http://insaniquariumguide.com/forum/ (go to the Plants vs Zombies section)
There's a pretty good chance ya'll already know about that forum already, but I figured I'll post the link anyway. There's an executable around there that will extract all the stuff from the main.pak file for you. It did forget to finish naming certain files in the "compiled" folder, so you'll have to complete those files if you want the game to run properly with mods.
Mods are possible in a way, you essentially just replace your custom images with the images from the game. Which is pretty good progress. You're going to run into a "compiled" folder with files that have the "compiled" file extension. These are most likely some kind of action script info or something, but not quite because certain name changing makes things appear normal. Further modding could be possible once those files are cracked and understood.