I've read that somebody was able to get usbNetwork hack on 2.5 by doing the following:
1) install usbNetwork hack before upgrading to 2.5
2) install 2.5 update
3) usbNetwork remains active
To use this way you need to have 2.3 firmware package and downgrade Kindle to 2.3 with it first. (I'm looking for the file now)
Also there is another possible way:
There is already jailbreak hack available for 2.5 - I've used it to install Cyrillic fonts. Instructions for this jailbreak hack tells that there is also possibility to install other hacks, but they should be recompiled for 2.5 firmware.
http://projects.mobileread.com/reade...-jailbreak.zip
BTW looks like there is also 2.3 firmware
http://projects.mobileread.com/reade...rkupan/Kindle/
Just found usbNetwork hack created specially for 2.5 (but author didn't tested it)
http://www.mobileread.mobi/forums/sh...t=86294&page=2