The only product that is going to have issues with this is MitelMail, no idea yet as to what Mitel's plans are for it.
Here it is on a product by product basis, as far as I see it:
3300: does not follow DST on it's own, always updated by human or OpsMan
OpsMan: follows Windows time on server, sync server to a NTP server on the I'net
200: specify fall back and move forward day in Form 4
Teleworker/ME/NuPoint 9 & 10: all run on Linux MAS server, can be set to sync off a NTP server, so should follow DST just fine if you do that.
PrairieFyre: windows box, see Ops info above
YA/Collab server: Windows box, see Ops info.
Am I missing any? That's about all the products I see on a daily basis.
MitelMail/Nupoint 7 (maybe 8.x as well, not sure): all have built in calenders which will not know about the DST rules. Upgrade to NuPoint10, or, remove DST update in the NuPoint/MM and set it twice a year manually. I don't think you'll see a patch come from Mitel for it since they are old systems.