Now no more wait, Microsoft has announced the imminent availability of public beta version of Service pack 1 for Windows 7 and Windows Server 2008 R2 at TechEd 2010 annual conference held in New Orleans, Louisiana, USA (June 7-10 2010).

Today, at TechEd, we are pleased to announce that by the end of July we will be releasing the public beta of Service Pack 1 for Windows Server 2008 R2 and Windows 7. - Microsoft Corporation

According to them expectation seems that we will be having SP1 Public Beta in the next one or two weeks.

Current assembly build tag for Windows 7/ Server 2008 R2 SP1 as follows :
6.1.7601.16562.win7sp1_beta.100603-1800 actual build in win7sp1_beta branch
6.1.7601.16561.win7sp1_beta.100527-1447 previous build
6.1.7601.16560.win7sp1_beta.100524-1800 previous build
6.1.7601.16559.win7sp1_beta.100506-1349 previous build