I don't know why, but there many reports on this forum (and other forums also) of "no booting anymore" after installing u-boot (in particulary Matan's version).

And all peoples with problems have in common that they own a Hw rev different from 2101.

I.e. Matan's version works perfectly on my N900 (rev 2101), but it hangs at "loading/starting kernel" message on some N900 of my friends with different revision (i.e. rev 2204).
And if I remember correctly there was some similar (no boot) problems reported on your first version also