Did you shut down slimserver before changing firmware and were you prompted to upgrade the firmware on the Squeezebox after doing so? (On the SB2 you are prompted to hold down the brightness key to upgrade, I'm guessing it's the same for the SB3). As far as the ip address, normaly the SB defaults to obtaining one automatically. You can find your servers ip address, and other required info for using an static ip by typing "ipconfig /all" (no quotes) at the command prompt on your computer/server. It is highly recommnded that you set static addresses for your SB, computer/server, and access point. To reset your SB unplug the power, plug it back in and press "add" on the remote. Also,check your MAC address under information->player information. Make sure it matches the MAC address printed on the lablel on the back of your SB. A bug in the 15 firmware can cause your players MAC address to change.
A lot of info but it covers most anything that I can think of that might be causing your problems.