SIP Trunking & the IP Contact Center – A No-Brainer?

As I talk to contact center customers, the topic of SIP Trunking does not come up as often as I would expect.  The benefits of converging voice and data onto common all-IP connections to the carrier seem pretty clear to me: reducing recurring costs of separate PSTN and data circuits, eliminating over-subscription of voice channels, reducing toll charges, gaining flexibility in call routing, and more. 
IP Contact Centers without SIP Trunking are realizing a solid return on investment by lowering administration costs and the costs of calls made within the enterprise as well as significantly increasing redundancy and flexibility in architecture.  Yet it appears that even more benefit could be gained adding SIP Trunking to the mix.
Is your organization considering a move to SIP Trunking?  If so, what are the primary drivers?  And, if you are staying with traditional T1/E1 spans, what are the reasons?
Let me know.