The general consensus is that a magic dagger (or other thrown magic weapon) can be used to deliver multiple attacks as part of a single action. I believe that is also the position of WotC CS. There may also be a FAQ entry on that.
There is a bit more speculation when it comes to non-magical daggers. Obviously you would need enough ammo to make the required attacks (1 dagger per target) but it is unclear if you would need Quick Draw as well if you have more than 2 targets, or if you can draw the additional daggers as part of the power, or if you can hold several daggers in hand at once for purposes of this type of attack. I believe the common conclusion is that drawing additional ammo is considered part of using the power. There may also be some CS responses on that as well, or a FAQ entry, not sure.
As a side note, the entire REASON for the 'magic weapons return' rule was to make thrown weapons a viable power delivery mechanism. Otherwise bows, etc are simply vastly superior for the purpose and builds relying on thrown weapons really aren't viable. A character relying on thrown weapon power delivery in particular would be well advised to take the Quick Draw feat however, since that will certainly shore up your case for what you want to do quite a bit, and it isn't a bad feat to have in and of itself.