I'd take a wait and see approach to the PS3 as right now there aren't any real killer apps and if you want to buy movies on Blu-Ray, you're gonna shell out $25-30ish a pop. If you have the extra $$$$ to spare and don't mind doing that sort of thing, it's not a bad deal, but still, you're getting rid of your PS2 and GS credit and STILL have to pay $300 plus tax for it. That's STILL ALOT of money/credit to shell out.
I mean, as 62t said, you won't get that much for a PS2 after the trade deal ends, but look at what else you have to give up WITH it for that $100. I believe the deal is 2 controllers(and I think they said Sony brand only), the system and a memory card. I'm not sure if it said anything about including games though. Think of it this way, they sell the system for $100, they sell the controllers USED for $19.99 and I believe the memory cards go for the same.
So, they're ass raping you and making $40 on that 'deal', unless of course you wound the wires around your controller and they have the infamous detached wires that makes them unresponsive about once every 30 seconds, thus making them completely useless for anyone.
Either way, it IS your decision ultimately, but this is NOT a 'deal' imo. Now, if you found out trade values for FYE and found out which last gen games got $10+ per copy and had enough FYE's around to load up with enough for a PS3's worth of credit, THEN that's a deal, especially if those games were bought from GS for $2-3 each.
I did that to get a PS2 and I'm just glad I kept going out of state for work alot, so I got to hit new FYE's and build my credit like crazy.
Again though, final decision is down to you, but I would honestly take a wait and see approach considering their PS2 failure rate last gen. I'd see if the PS3 really IS as 'well made' as everyone claims it to be.