One of the things that always amazes me in technology is how people seem to think that technology revolutions have suddenly stopped because they've found a hammer. Whether it be Web Services, REST, EAI, ERP, Excel or anything else certain classes of IT people seem to become blind to the history of IT and think that because they are happy with what they are using that there will be no other developments.
As an example of this lets look at REST (as implemented by HTTP) and SIP, now REST lays down a bunch of constraints and has its HTTP implementation as the way to do everything. HTTP is of course limited in what it can achieve and certain tasks it just doesn't seem to be the right way to go, VOIP being one example, now SIP provides some similar elements in terms of philosophy of simplification (which is nice) but enables multiple protocol interaction and can (in theory) support application to application communication.
Now I've been looking at it and I'm not sure why you couldn't use SIP as a boot strap before electing to use HTTP, VOIP, Video or something else for communication between applications and from applications to people. After all wouldn't it be nice for the application to send you an SMS when something changes rather than polling a web server? These people talk about using SIP to kick off VoiceXML as a reasonable approach today, and VoiceXML is an HTTP based standard.
This would of course mean that HTTP, and potentially REST, would be relegated to specific application problems with an increased requirement for sophistication around the actual mode of interaction between systems and between systems and users.
The real point here is that no one technology will ever remain for ever as the pinnacle of achievement. It is either replaced (e.g. CORBA to Web Services) or commoditised (e.g. TCP to HTTP) and in both cases it ceases to be the cutting edge of IT delivery.
Will SIP relegate HTTP to "one" of the options? Who knows. But what I do know is that backing HTTP to be the cutting edge of IT delivery for the next 20 years is on a hiding to nothing.