I spent a bit of time last night ruling out everything I could that could be causing my problem, namely, sending WOL requests to clients to wake them up.
Uninstalling SP1 and all MS updates for Vista broke ZA (BSOD on Vista logon) so I had to fire up in safe mode.
I noticed that when I did this (safe mode with networking) I could sucessfully wake PCs up from my Vista machine.
after reinstalling all updates and SP1 ZA was still broken so back to safe mode and I could still wake my PCs up.
So, I uninstalled ZA which then let me boot up Vista as normal and everything was ok.
As soon as I reinstalled ZA I'm back to the WoL packets being blocked again.
At all times incoming WoL requests wake the Vista PC so this is not a problem (however this is to be expected as the PC is not physically running any apps when it's in standby).
I've tried turning the Internet and Trusted zones to off too but this still does not let the packets out.
All my PCs are in the trusted zone.
Is this a known issue?
I've seen that there are beta versions of ZA for Vista but I'd rather wait for a stable release or possibly uninstall until this is resolved.

Operating System:Windows Vista Ultimate
Software Version:7.1 (Vista)
Product Name:ZoneAlarm (Free)