I finally brought my N900 back to life after having not used it for exactly one year. Reason was, I think, I had the same problem as you. Actually, I was in the rush to install multi-boot so I did a picked the multiboot app from the app manager thinking I'd be able to perform multiboot without needing any other files. Then I was left with a boot looping over and over. I couldn't get it back until tonight when I finally decided to try reflashing it once again. This time, I made sure I was on Windows XP (32-bit), battery charged (just good enough), and all the files I need (flasher, firmware).
I flashed the FIASCO way a couple times. The first time didn't work when I used the global image, but then I flashed again using the USA image, and somehow it magically worked! I was a little bit skeptic while waiting for the screen the loading upon bootup, but after about 5 long minutes, I finally see the Nokia hands animation, and then I threw myself in excitement! I can't believe it; I'm just so damn happy right now. \/
Maybe you should try the other firmware images?