There are options to sign up to test the game on either PS3 or PC. You'll need one of SOE's Station accounts to put your name down for the PC version.
There's a pre-order page open on the DC Universe site too, although at present pre-orders are only open in the US and Canada. (And while Germany, France, Spain and Italy are represented as "coming soon", the UK doesn't appear at all. What did we do?)
Pre-order bonuses include in-game weapons, a comic, a Batman figurine and an arena battle in which you can play as Batman. There's a collector's edition of the PC version of the game, too.
DC Universe Online is due out in November, and it's the last MMO still claiming that it will make a simultaneous release on console - PS3 in this case - and PC. Will the PS3 version befall the same fate as Final Fantasy XIV, or will Sony push it through?