So, first needed to backup some things: AptBackup, SpringBack and finally, a backup with iTunes.
Then create a custom ipsw with Pwnagetool. To install this ipsw, I put the iPhone into recovery mode (did not need the DFU mode, since it was already hacked) by holding both buttons on the iPhone and release the sleep button when screen goes black. Installation went fine, but...
AptBackup icon was hidden because I used the Categories app. No sweat on that, but after unhiding, AptBackup failed on me since it wasn't tested with 3.0 yet.
Solution: Installed OpenSSH and APT again and ran the following command:
for p in `cut -f 1 /User/Library/Preferences/aptbackup_dpkg-packages.txt`; do apt-get -y --force-yes install $p; doneAny packages not for 3.0 won't be installed (like mobileinstallation patch), so you need to find a newer version for them.
I seemed to have lost most settings of Cydia apps since iTunes only backs up managed settings. This includes categories, spbk files, whatever. The only items in /User where Applications, Documents, Library and Media.
So next time, I'll copy the other items manually. Also /Library/Themes for custom themes.
Tip: Get Macfusion/FUSE to mount the iPhone volume on your Mac. Or use Cyberduck.
Replace yellown0w with ultrasn0w if you updated the baseband firmware (Pwnagetool does that by default btw!). At the time of me writing it wasn't released yet though... oops.