Mitel has already purchased Aastra, a leader in SIP devices outside of the Americas... buying Polycom, which is a market leader in SIP and conference devices in the Americas and Russia, is only a natural progression.
Like it or not, SIP is the foreseeable platform for the near future (10-15 years at least) of the telecommunications industry... we are already seeing significant enhancement in SIP device capabilities in the various Mitel platforms, and that will continue for some time.
I am excited... Mitel has always been a significant market leader in hospitality and a few niche markets, but in general IP/PBX and UC Collaboration sales Mitel is a pretty distant 4th behind Cisco, Avaya, and Microsoft worldwide, with Shoretel and NEC nipping at their heels. The acquisition broadens their market-base, and most corporate people know and trust Polycom, 98% every conference room I see has a Polycom phone in it.
I see this acquisition as a win... unless Mitel decided to do something totally stupid like dissolve Polycom, I don't see how this could be a bad thing.