The initial phase of the beta for StarCraft 2: Wings of Liberty will come to an end on Monday, the 31st May.
Once that happens Blizzard have said it will be offline for a few weeks while they make changes. The beta will go into its second phase two weeks before its full release on the 27th July.
Blizzard said on their official forums:
“We’d like to thank all of our beta-test participants for your enthusiasm, dedication, and valuable feedback during the beta test.”