Elves: +2 Dex, +2 Wis
Eladrin: +2 Dex, +2 Int
Drow: +2 Dex, +2 Cha
Those I'm pretty certain about (the first being confirmed, the second implied, the third logical and going well with legacy presentations of the drow).
Dwarves: +2 Str, +2 Con
seems almost mandatory. +2 Con, +2 Wis would be my second guess.
Halflings: +2 Dex, +2 Cha
Wisdom is useful to rogues, but Cha is too, and I don't see halflings as great clerics. It also seems inconceivable they would get the same bonuses as elves, and they just aren't tough enough to warrant +2 Con at that size.
Dragonborn: +2 Str, +2 Int
This goes with the whole draconic thing. It's reasonable to suspect that dragons are not major spellcasters in 4e, but some do cast spells. This would make dragonborn a natural fit for tactical soldiers or warrior-mages, which seems to suit their schick.
Humans: +2 Str, +2 Cha
As strong as dwarves, charismatic, basically just a dominant kind of nature. This harkens back to AD&D, when only humans could get an 18/100 Strength and even female humans could be a lot stronger than female demihumans.
Tieflings: +2 Dex, +2 Cha
This will make them "Great warlocks," and will give them different stats from the eladrin. They have the same probably stats as drow, but since drow probably won't be in the PHB and have very different racial traits, not a problem. +2 Dex, +2 Int would be my second guess.