When you assign a Reboot To task to a known system, an SBS file is created on the site server to tell the server that system x is booting to automation. That updates the PXE server service to know what to do with your client system.
You can verify that this file gets created by looking at the SBS Store folder within a minute or less after assigning the job.
The catch? Well, be sure you're booting to the right PXE server. If you have a Site Server, then you have at least 2 task servers: your Site Server and the NS. The SBS file is sent to the Task Server to which your client is used to checking into, not to both! If you don't see the SBS file on one of the servers, check the other.
If what I've just said confuses you, you might like a quick review of the DS 7.1 Architecture and Dataflow document posted both in Connect and in the KB. It talks about these files. You can find them in the \\server\deployment\task handlers\SBS\SBSStore folders. IF they're in the SBS folder, then the services aren't working. Anyway...
If the SBS file is getting to the wrong "PXE" server, then we'll need to modify the query that tells us which PXE server to send the file to using TECH133468 found here:
http://www.symantec.com/docs/TECH133468
For small-midsized organizations and single NS's under say 15K nodes, I'd recommend the full envirnoment script.
Anyway, see if the above info helps at all. GL!