This is kinda tough, and I was in a similar situation - had been using a 13" MB with 6GB RAM for a few years, with external LCD. It was enough to run a VM or two, but the screen res drove me insane when I had to present occasionally from it and it wasn't scaling an external projector nicely - presenting in 1280x800 = meh. :-/ For me, it was mostly quick enough, mostly portable enough, but USB gave up the ghost on it, so I needed a replacement *now* as I use a USB based security device, and do a lot of skype calls and usb headsets in general work better than line/in and out under OSX in my experience (never sure why, but it seems to be so). I picked up a very lightly used 2011 MBP, 2.3/quad, AG, 1GB VRAM/hi-res, 7200RPM BTO, basically the highest end 15" available sans SSD. It's a *nice* machine, and SNB is quite a bit faster than the C2D, while within the entire run of C2Ds, there wasn't really much 'justification' to upgrade, except for max RAM supported (vms..).
I would have probably preferred to wait to see what the next refresh brings, were I able to, simply because I don't think it's that far off, and while yeah, I do use the extra 2GB of RAM, and like the higher res, that will certainly be there on the next gen. The only thing I'm not liking on the new/current model is the heat dissipation - with an external display connected, my 'idle' (I don't shut down or reboot daily, generally it's my desktop replacement, so stays up a week or so if not longer at a time, the usual apps - eclipse, multiple browsers, docs in various formats, Mail, sometimes with a DB or other server running, etc.) is generally 70-75*C, fans at 3k or so, sitting on an elevated stand with the rear raised up further. If the next gen adds USB3 and lowers temps/improves cooling, or adds a second TBolt port and we start seeing use of them (good bandwidth, just not much to plug in at the moment of use), I might be kicking myself a bit. Of course, if the next gen/refresh just removes the optical drive to shave a few pounds, and gains some 10-20% performance - it's an overall wash/no real difference to me except loss of a slot for optibay.
If my MB hadn't been ready to give up the ghost, and I was buying 100% out of pocket on a brand new system, I would have personally tried to wait until Ivy Bridge, as we should get USB3, PCIE3, possibly lowered temps, and likely whatever the 'next form factor' is coming from Apple. As it is, I tend to try to keep my systems for at least 3 years, will move to SSD and optibay within a year, and go from 8->16GB once prices drop, and assuming thermals don't cause these systems to have a shortened lifespan, the only thing I think I'll likely be really 'missing' is USB3, a bit of weight at the benefit of optibay instead (a win for me, maybe not for you) vs what I expect released next.
Now, as you said you're coming from ultra-portables, I agree - an MBA is cool, but likely can't yet really replace the Pro based on max RAM alone. The next gen pro *may* well get you there - an 8GB 13" Air with a storage bump, with enough dongles (gigethernet mainly, no 10/100 please) could be pretty compelling even vs my 15" quad monster, to me - but it's not here yet/not 'enough,' and it seems not for you either, so it's down to waiting to see what the next gen Pros bring.
Anyway, sorry for the long post - it irks me when people are anxious for others to buy what they did without always considering it's real $ and someone else's needs may not be the same as their own. If you move from C2D to current i5/i7 - the speed improvement is insanely nice, and I'd consider bringing your Sata-II SSD over to it - yes, it will benchmark slower vs a Sata-III, but will you really notice it in most cases? Likely not except for virtual d1ck comparisons...and now that the 13" i7s are available, the C2Ds are already taking a fairly big $ hit.
You didn't mention how 'portable'/mobile you really are day in and day out - while I'll admit the thought of an 8GB 256GB SSD MBA might have made me consider one over my quad monster 15", I think you'll adjust pretty easily mobility-wise with your C2D 13" Pro, even if occasionally you were wishing it were slightly lighter/sleeker/etc.
