Hi
Please check the software compatibility link for BE 12.5
ftp://exftpp.symantec.com/pub/support/products/Backup_Exec_for_WindowsNT/307063.pdf
This was created on Created on May 21, 2009 . But still All MAC OS X 10.5 patch levels are supported but only up to 10.5.5
Usually compatibility list are not very frequent updated . You can expect the next version of backup exec end of this year and early next year. But the date has not been released yet.