Randy Moss is in the final year of his contract, likely a big reason the Patriots were willing to deal him for a third round pick just four weeks into the season, especially with a lot of unknowns regarding the CBA and an NFL season in 2011, with the team unsure if they would receive a compensatory draft pick.
Originally, the deal was contingent on Moss signing an extension with Minnesota, but then the team shifted its focus to simply just getting the trade done, choosing to worry about getting Moss signed to a new deal later. And indeed, Wednesday's trade was completed without a new contract for the wide receiver.
More than likely, the Vikings will be talking to Moss about a new contract very soon (if they haven't already). It's highly unlikely a team would give up a third round pick without some sort of guarantee that it will last longer than one year. Jay Glazer reports that the Vikings just wanted to get the trade done so he could be in-house for Monday Night Football this week. Presumably they'll be focusing on a contract after that.