very heavy? do take a look at the canon/nikon 70-200 f2.8 (equiv to 35-100 on a 4/3), they are just 200g lighter but both are f2.8. plus i have yet to include the additional weight of the lightest 35mm FF dSLR weight(in this case probably the 5d) over 4/3 dSLR.I do hope they can get clean ISO6400 from aps-c or 4/3 sensors. I really don't relish the thought of carrying those huge full frame lens on my trips.
For my needs, the low light situations happens indoors (homes, restaurants, pubs), so I don't need huge telephotos, medium range ie. 50 -100mm is good enough for me. If the full frame sensors gives clean ISO6400 images, heck, I don't even need F2 zoom lens, F3 or even F4 would be a good compromise against weight and cost.
As for the 35-100 F2, don't drool yet, check out this site:
http://www.e-fotografija.si/templates/default.aspx?a=1071&z=93
it is a monster and it's weight will make your arms tremble, negating the F2 aperture advantage. Wide open, it is very soft, compared to F8. Most lenses do not perform their best wide open, so I try to use the sweet spot, somewhere in the middle and gives me good sharp focus everytime, even if I fudge the focusing (bad eyes you know). Of course, if you want really shallow dof, that's another matter, and it comes at a price (ie. soft images)
and if a 4/3 can perform at iso1600, with a f2 lens it is almost equiv to a 35mm FF giving iso6400 shots with a f4 lens. do rmb, f2 is 2stop faster than f4.
anyway the choice is urs, if u do think 35mm FF advantage is really so huge, go ahead and upgrade