As promised, here's a list of codes to unlock 9 new characters, 3 alternative outfits, and the unlock code to the Saturday Night's Main Event arena. Keep in mind that these codes are case sensitive, so be careful when entering them in.
Unlock "Nature Boy" Ric Flair - FlairWoooooooooooooo
Unlock The Boogeyman - BoogeymanEatsWorms!!
Unlock Curt Hawkins and Zack Ryder - Ryder&HawkinsTagTeam
Unlock Tazz - UnlockECWTazzSvR2009
Unlock Snitsky - UnlockSnitskySvR2009
Unlock Layla - UnlockECWDivaLayla09
Unlock Jillian Hall - PlayAsJillianHallSvR
Unlock Hornswoggle - HornswoggleAsManager
Unlock Vince McMahon - VinceMcMahonNoChance
Atlernate Costume for CM Punk - CMPunkAltCostumeSvR!
Alternate Costume for Chris Jericho - AltJerichoModelSvR09
Alternate Costume for Rey Mysterio - BooyakaBooyaka619SvR
Unlock the Saturday Night's Main Event Arena - SatNightMainEventSvR
One word of warning -- these codes have not been tested (some of us want to earn these things the old fashioned way). That said, considering the source, they should work perfectly.
Playing through the various career mode paths also result in different titles and match types being made available. For example, winning the ECW title in this mode unlocks the Extreme Rules Match, and becoming Intercontinental Champion opens up the Steel Cage match.
Playing through Career Mode is the only way to unlock the WWE Hardcore Championship and the Cruiserweight Championship, a pair of defunct belts no longer used by the company but mercifully still included in Smackdown vs. Raw 2009. Earning these unlockables is pretty much self explanatory -- just keep winning matches and titles and eventually you'll earn all there is to earn in career mode -- but it can be the sort of thing that many players would overlook in their quest to open up all the game's content, so it's included here more as a reminder than anything else.
Have you unlocked everything there is to unlock? Try the new WWE Legends of Wrestlemania for more wrestling fun.