Same problem here and have gone thru the same steps Polar WSU has and the solution from the HP Tech.
Same thing - the HP 8600 shows connected to my wireless network both at the printer and from my router software and even thru Windows Explorer (network) - IP address is correct - turned off my firewall... still nothing. Tried shutting everything down -waiting - then starting the router, printer then PC. No help.
Trying to print using MS Word, Notepad and even an Adobe Acrobat file and it still says Printer is of line. Getting "VERY" frustrated..!!
Any help would be appreciated. This printer has worked fine for several months - and nothing has changed in my system or network.