slugg,
Planetside is still running today, however there hasn't been a patch since 2006/2007. It does not run in Vista. Since it's not actively being maintained or expanded, population has dropped, mostly because people can't run the game.
SOE business model is to pump whatever resources into whatever games that are making money. The more subscriptions = more resources. Since they have not had any resources on Planetside in years, my guess is that the game has only enough subscribers to keep it afloat (pay for servers, bandwidth) as they have basically nobody supporting the game right now.
My guess is it will be on the chopping block soon.
Planetside was indeed fun at the time. I played for awhile but quit, not because the game was bad by any means, but because I don't believe they understood balance. Which frustrated me.
I remember firing my anti air gun at a flying craft and unloaded nearly ALL my ammo on it, and they turn around and shoot me with one missile and I'm dead. There was other similar issues with the game, it was like you were firing blanks alot of the time. However I loved the concept of the game and it was fun, provided you knew what you were doing and played the right things.

But the mech warrior guys ruined it with the expansion. That made me quit.
EDIT: The link provided had the message:
We were able to speak with John Smedley tonight CEO of Sony Online. While we could not get any specific information about the all-but-confirmed PlanetSide 2, I was able to get some feedback on the current PlanetSide:
1. A new patch is coming.
We're going to be fully Vista compliant among other things and bug fixes / anti-hacking work.
2. Planetside fans will be happy We have big things planned in the next 12 months. We're working on something new that's really cool.
Some awesome news for the current game!! As always PSU will be your #1 source for any new and upcoming information about PlanetSide 2 so stay tuned!
Sounds like Planetside is doing better than I thought.