According to Network World, Sam Houston State University is in the process of mass-dumping CallManager and Nortel PBXs in favor of Asterisk running on Linux:

SHSU so far has moved 1,600 IP phones from Cisco CallManagers to Asterisk, which runs the IETF-standard version of SIP. The Asterisk functions are spread across six redundant Dell servers: two act as redundant PSTN gateways (and are outfitted with four-port T-1 cards from Digium, which commercially distributes Asterisk); two more servers handle call processing; another set provides voicemail.

I’m sure some in the open source community would take issue with Network World’s designation of Asterisk’s SIP implementatin as “IETF-standard”.

Still, it’s good to see Asterisk making more enterprise-grade inroads.