You need to think long and hard about the black swap issue; it's not just a matter of convenience, it's a matter of flushing $60-70 of ink away when you swap blacks on the 4000/4800 based on the estimates I've seen.
It's a shame the 3800 doesn't have at least basic roll paper support, this decision would become so much easier IMHO.
I think the chances of a 3880 in the next 2-3 months are slim to none. Maybe in 2008....
Agreed on the black ink swap. The 4800 uses less ink than the 4000 did, because it flushed all 8 lines and the 4800 only flushed 4. Estimated ink for a swap on the 4800 is around 90ml, or around $40 in ink. So the round trip is about $80. I read somewhere, but can't find it now, that the 4880 now only flushes 2 colors instead of 4, which would cut the cost to around $40 for the round trip. Still quite a big higher than the 3800(which I believe is around $5-6 for the round trip), but 1/2 the cost on a 4800.
One other advantage of the 4880 is it will support 16bit drivers under OS X 10.5. I'm not sure that will translate to any visible difference although many canon users claim they can see the difference with Canons 16bit driver.
I think the new head technology, new screening technology, and especially the new inks make the 4880 the machine of choice if selecting between the 4880 and 3800, unless you need to switch blacks more than occasionally.
I tend to agree with you on a possible 3880. I would be surprised to see it before PMA, and not even sure it's anywhere in sight. Who knows, they may not even plan on doing it.