Interesting little tidbit I've heard, though I'm not sure if it's verifiable, but I remember reading it in a few places that had some decorum to 'em.
According to Federal Law:
You can get charged for having a plant.
You can be penalized for having a seed.
But they can't do shit if you have an unrooted clone. Apparently, according to the Federal statutes, in order for it to be considered a plant it has to have:
1) foliage
2) roots
Kind of amusing if it's true, eh? There are a lot of little loopholes like this in various laws, so I'm inclined to believe that it's actually true.
And for the record, yes, state laws vary, and if you're caught by the police, that's how you'll be charged. But there are Federal penalties as well. You're only charged with these if the arresting agency is a federal agency, such as the DEA. And from what I recall, they're rather draconian. I mean... look at the penalty Timothy Leary got for having a single roach! Though I'm sure they've been amended since then.
If you're looking for the federal penalities, there's always Erowid. I wouldn't be surprised if the penalties themselves are listed in the the Controlled Substances Act, or other similar statutes themselves. The DEA site is probably also a source of information in that regard, though I would personally be wary poking around in there, if I were actually growing.
One might also want to check NIDA. You know, the people who supply marijuana to researchers but meanwhile won't allow anyone access to it unless their study is specifically aimed at upholding prohibition.
~Ethno