IMHO, a lot of interest in the forums could be drummed up, by simply telling people how useful they are. Maybe a short 1/2-paragraph blurb, under the mainpage's link to the forum, could help new players overcome any shyness/paranoia about signing up?
After all, this is one of the more supportive and civilized web forums out there - if people knew how awesome it is, without having to first sign up (anathema to many casual flash gamers), there may be more activity.
On a side note, it was mentioned in another thread (sorry, too lazy to find it right now - it's early, here!) that it might be a good idea to gather a few veteran players, as volunteers, to guide newbies through deckbuilding, synergy, and basic E:tG concepts. How about a new PvP (PvP0?), where veterans hang out, giving tutorials to new players? This would also provide excellent opportunities for the forum vets (so many people with thousands of posts! :envy) to show that they're gamers, like any newb, rather than an elitist circle of injoking, newb-hating d-bags (which, for the record, people here most certainly are not), and to tell players directly about the benefits of forum membership - heck, most of the fun of the game comes from various forum-based PvP events, of which new players know nothing.
Please forgive the disjointed thoughts - morning coffee isn't working properly yet.