It's there as a warning, but if you understand the risks and want a quick way around it, you can edit the file /usr/pandora/scripts/op_cpuspeed.sh and change the safemhz value to 800 or something. I'd put it at the value you know to be safe on your own console at OPP3. That way if your finger slips and you accidentally set it to 900 or something which would normally crash your Pandora, it'll come back with the warning and give you a chance to fix it.