Edit: Here's the info from Wacom.
"A. The Wacom Serial tablets can be used with a Serial to USB adapter in operating systems up to 9.2.2. Wacom has tested the Keyspan (www.keyspan.com) Serial to USB adapter. If you have a Macintosh tablet with an ADB (Apple Desktop Bus) interface (the tablet model number will end in "A"), the iMate USB to ADB adapter from Griffin Techonlogy (www.griffintechnology.com) will work to adapt the tablet to USB. .
NOTE: OSX requires a USB tablet. ADB and serial models will not work in OSX.
"A. The Wacom Serial tablets can be used with a Serial to USB adapter in operating systems up to 9.2.2. Wacom has tested the Keyspan (www.keyspan.com) Serial to USB adapter. If you have a Macintosh tablet with an ADB (Apple Desktop Bus) interface (the tablet model number will end in "A"), the iMate USB to ADB adapter from Griffin Techonlogy (www.griffintechnology.com) will work to adapt the tablet to USB. .
NOTE: OSX requires a USB tablet. ADB and serial models will not work in OSX.