@Soulfish: There are companies that will provide Asterisk support which will mitigate the risk for your SLT. As for the Mitel stuff... it's ace! Cutter Project partner up with people to provide unified communications solutions so would be able to put you in touch with a partner near you (I always think a 'local' company is best for phone support - it's no good having to have an engineer drive 4 hours to get to you!!!).
@John: Never going back to PMP and XenServer is still nicer to administer than Hyper-V! :p
The whole reason for considering OCS is because of the need to use a subscription licensing model to get Windows 7 Enterprise. Since you get loads of random CALs with these models, I may as well try to utilise them.
Ah gotcha :) Exact same reason why I use a very large amount of MS, also because I like it anyway but I do the year licenceing so get a pile of others as part of it that I might as well use :)