Turtle, implementing it in Core would still constitute a hard fork, between people using the post-raising-blocksize version and the pre-raising-blocksize version.
A hard fork is absolutely inevitable, because the 1MB cap is not sustainable. It's fine for *this* year, and probably most of *next*, but the bitcoin economy simply can't grow enough if the limit is not raised eventually. I'm 95% certain that Gavin is 'on the right side of history' here.
The fork is inevitable, and IMO so is a short period of uncertainty and hence price stagnation (hah! how would we tell?
). What's uncertain is just how disruptive that will be. But I'd be very surprised if 'Gavincoin' didn't emerge as the clear winner after, oh a week, two weeks maximum.
I have heard no convincing arguments for keeping the blocksize at 1MB. But to be fair, I've pretty much only heard from Mircea Popescu, his sockpuppets, and his sycophants. Peter Todd sounds like a reasonable guy, can anyone point me to a long, carefully argued screed from him?