Okay, so forgive me if this is going to be a long opening forum post, it didn't seem right as a blog.
At the beginning of the school year, my teacher had us learning about poetry and analyzing, it, which involved a lot of studying symbolism, figurative language, etc. And recently, I've had to analyze some artwork (Mostly Sistine Chapel stuff) for influences in Greek myth. So the reason I found this out is because while I was playing GW my brain was still in that "Analyze for symbolism mode." Here we go:
I believe that PvZ, most notably GW, is a metaphor for the state of the environment compared to things like industry. Let me list down my evidence because that's easier (both for me and you) than writing an essay.
1. Plants = nature, environment-friendly actions, etc. Obviously, this is because plants are natural. I mean sure, you have stuff like the plants in Far Future who are partly mechanical, but they are still works of nature and can grow like any others. There's other stuff too, but that comes in later.
2. Zombies = pollution, industry, etc. This takes a little bit more of explanation than the plants. Firstly, they are undead, half alive, half dead, similarly to how some people I've seen refer to the Earth as being half dead due to pollution. Next thing, how the team is represented. On the Zombie side of the base, all you see is machinery, smog, dead trees, and the like. Generally, places where industries have been are depicted in the same way - smoggy, dark forests with dead trees and factories (the mechanical thing) in the background.
3. Materials used for devices. Most of the Plant gadgets are either al natural, natural with a mechanical twist, or made from junk completely. Meanwhile, the Zombies have brand new, pristine materials for their devices. Take for example the Infinite Robots  (and no, the following names are not official, I'm using them for convenience). The Dinobot (Plants) is made totally from junk somehow arranged so that it can travel to alternate dimensions and be combat-worthy. The Cattron (Zombies) is pristine, with shiny metals and that "brand new" look. These two types of design allude to the materials associated with environmentalist/industrial things. Those who want to go green try to recycle what they can, to reuse their "useless junk" and make it new. Those in the industry need new materials, and although many industrial things are made form recycled materials, they don't usually say that. In the same way, Dave takes whatever he can find to make his stuff, while Zomboss pulls from his newly made manufactured things.
4. Locations associated with each class. When you think of the Zombie maps, you'll usually come up with the Z-Tech Factory or Moon Base Z, both involving highly technical design. Plant maps generally look "natural," with trees and a lot of green, like Garden Center or Wall-Nut Heights. These designs reveal what each side is all about. Zombies are about manufacturing and stuff, which is why they mainly have factories and things. Plants are all about staying natural, and living in harmony with nature (hence the suburbian areas).
5. The leaders. Zomboss and Dave are pretty much worlds apart, and they kinda represent those who you assume to be on each side. Dave is your average guy (aside from his mental state), who even when fighting an all out war prefers his base to be his comfortable, familiar suburban house (despite the thing being on top of a giant tree). He dresses casually, with a white T-shirt, jeans, and his pot which may as well be a baseball cap. Meanwhile, Zomboss wants to live in a mansion as big as his ego (*insert Wombo Combo audio here*) and always dresses in his snappiest lab coat and gloves. Zomboss could be equated with big business owners who always think about money and profit. Their personalities match too. Dave is mostly humble, but passionate about what he's fighting for. He is willing to risk himself (riding Penny of course) to win. Zomboss on the other hand will only go out when Dave is around, and even then moslty stay at the back, letting his Zombies do most of the work. He's not willing to sacrifice himself to get what he wants.
There's my evidence. If you have questions about it, ask, because I have some more stuff in my brainz too. I just couldn't put it in (as I'm writing this I'm kinda pressed on time). But I'll return later to add it. For now, here's my conclusion:
Popcap is sending a message to everyone playing to do thier part in saving the environment. They just make it so fun that we don't notice it.
What do you think? Go ahead and debate it. I'm here to jump in when necessary.