Looks like the ConfigMgr documentation team has been at it again, this time creating an awesome explainer for PXE cache expiration behavior in Configuration Manager 2007 SP1 and SP2. If you've ever encountered a situation where running a second OSD Task Sequences within an hour or so results in the second deployment being ignored and failing to start then this may explain what's going on. On the plus side, it also talks about a fix that's already documented in KB969113. You can read all the details of their post here.
J.C. Hornbeck | System Center Knowledge Engineer