Okay here it goes, in a nutshell...

Will they track you down based on your IP to determine that you've been growing?

Not likely

If you are busted for other reasons can they use this forum as evidence of your involvement?

Yes

The fact of the matter this site is dedicated to the medical user. Now not all members are in fact medical users, however, by in large the members of this site are very discrete and seeing as that is the case, i don't believe you have much to worry about.
good lord i have tried to say this many many times you can not be arrested for posting a photo of a pot plant i repeat CANNOT be arrested. at least in the U.S. they can and will track your activities before an arrest is made wich means they will supeona your bank accounts, credit card info and internet activity to show that you are indeed a grower then the D.A. will use that evidence to go before a judge and get an arrest and a search warrant. just keep your activities low key and for gods sake remember the 5 most important rules:
1. dont tell any one
2. dont use creit cards
3. dont TELL any one
4. DONT TELL anyone
5. DONT TELL ANY ONE