For those wondering:
Yes, N900 can multiboot into other OS'es
(disclaimer: this needs a patch to /sbin/preinit. Patch will be in bootmenu package or upstream when final SW is done. USB networking doesn't work yet.)
Mer runs fine. Same bootmenu system as on 770, N800, N810. And yes, Mer will have a N900 port. You can probably even clone Fremantle to SD if you so wanted.
Way to multiboot will be having keyboard out when booting and using cursor buttons to navigate.