In my last visit to 24C3, I also got one of 99 EUR NSLU2 Slug devices.

unslug Linksys firmware

sudo apt-get install upslug2

existing firmware version

sudo upslug2 -t 00:1A:70:95:AD:6D -v
NSLU2     00:1a:70:95:ad:6d Product ID: 1 Protocol ID: 0 Firmware Version: R23VA5 [0x23A5]

flash

Flasing has great user interface:

sudo upslug2 -t 00:1A:70:95:AD:6D -i slugosbe-4.8-beta-nslu2.bin    
Upgrading LKG95AD6D 00:1a:70:95:ad:6d
    . original flash contents  * packet timed out
    ! being erased             - erased
    u being upgraded           U upgraded
    v being verified           V verified 

  Display:
    <status> <address completed>+<bytes transmitted but not completed>
  Status:
    * timeout occured          + sequence error detected

  4605bf+000000 ...UUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUu----------------------------

First flashing didn't end up quite well, since I got blinking green/amber led (even after power cycle).

links