Most likely, according to rumours, it will be July 6, 2011 (Wednesday)
Source from T-GAAP.
“We’ve just received word from our source in the San Francisco Bay Area that Apple is planning to release Mac OS X 10.7 Lion on Wednesday, July 6th at 9:00 am Pacific Time.
One particular item of interest about this news is that with OS X Lion being an online-release-only, Apple can wait until the 11th hour to release new OS. They don't have to first lock it down, then press it onto DVDs, then box it, then ship to their hundreds of stores and through distribution in order to meet a certain publicly announced launch date. With an online release, Apple just needs to upload the final release to their servers and then make it available.”
UPDATE (Jul 3, 2011)
According to 9to5Mac, reliable sources said the Release of Lion might be July 14, 2011.