I have not got Valkryia Chronicles yet, but based on the demo, it's one of the best Tactical RPGs I've played. As soon as I can afford it, I'm getting it.
LittleBigPlanet is an obvious get; again, I've not got it myself, but it's on my list. Also there is Disgaea 3, I have to make sure to get it from Best Buy so I get the soundtrack with it. Folklore is the first "RPG" for the PS3... not a great title gameplay-wise, but it has an interesting style to it. Eye of Judgement is also essential for anybody who has ever once been interested in Magic: The Gathering or any of its other card-based brethren. And, of course, there is the reason I got a PS3, Metal Gear Solid 4. Uncharted: Drake's Fortune and Heavenly Sword are worth getting, but only if you can find them for really cheap; I managed to snag Uncharted for $20 at a pawn shop, but it regularly goes for $35. Heavenly Sword I rented (and beat in about 2-3 days), but I'd buy it if I found it for about $15.
If you wanna start off cheap, there's Motorstorm ($12), Virtua Fighter 5 ($13), The Darkness ($13), The Club ($14), Ridge Racer 7 ($15), Resistance ($19), Warhawk ($22), Burnout Paradise ($23), Devil May Cry 4 ($25), Folklore ($28), and GTAIV ($30). The only games I haven't tried out from that list includes RR7 and GTA4. Resistance, The Darkness and The Club come with my highest recommendations, despite the fact that they all belong to the overflooding FPS genre.