Small workaround to stay hacked in 3.1 firmware:
Before upgrade:
1. Install Jailbreak
2. Install usbNetwork
3. Upgrade to 3.1 firmware
4. Backup original file /etc/uks/pubprodkey01.pem
5. Rename pubhackkey01.pem to pubprodkey01.pem
That's all. Now you can install/uninstall any hacks (except Amazon's firmware, to install it you must revert original.pem file).
P.S. This is temporary solution

P.P.S. If you don't know what you doing exactly - do not do it, wait for new Jailbreak...