The first half, from 0xbfc00000 to 0xbfdfffff should flash without any problems.
So flashing u-boot should be completely save since it resides at the very beginning of the flash area and will never go to the second half of the flash.
Me and some other flashed u-boot using "run upgrade" from the RAM build without having problems.
When installing u-boot, will my current openlink still work and boot ?
AFAIK and based on reports from this forum, kernels 2.4 and 2.6 and all distributions based on them work with u-boot.
Problems arise if you would upgrade to Debian Lenny and still want to use kernel 2.4, but this is not in scope of the current OpenLink versions and has nothing to do with u-boot.
I need samba and nfs and something like avr_evtd daemon...
I´m using foonas-em only as an emergency system, but based on notes from http://www.foonas.org/
and this forum, everything you require should be possible. Personally I can confirm, that even foonas-em has avr_evtd support which means that the LS does not reboot after a few minutes. But currently there is a problem using the buttons (power button, reset button) of the box with kernel 2.6. This will freeze the LS immediately (not related to u-boot).