The exclusivity deal expires after 30 days, so PS3 and PC players won't be left out entirely. There's no word yet on when these add-ons will be coming, what they'll contain, or how much they'll cost. It's interesting to note that the announcement specifies that the first two packs are exclusive. If past Bethesda games are any indication, there's likely to be much more where that came from.
Before you start gloating, Xbox fiends, it's probably wise to make sure these add-ons are worth getting in the first place.