According to a rough schedule posted on debian-devel-announce #1 #2, Debian 5.0, codename Lenny, should be released in the second half of 2008 (probably around September).
Current status : Frozen
The purpose of the freeze time is to improve the quality of the archive by fixing RC-bugs.
(Basically, no new package and no new upstream version of existing package will enter DebianTesting. Only non-invasives changes are allowed if justified. read the freeze announcement for more information).