I just upgraded from 2007.0 to 2008.0 and same problem. I cannot print from the XP box, even when logged in as domain. I can install a new printer, but not print.
the smb.log shows for the pdf generator
[2007/12/20 20:14:26, 0] lib/system.c:set_process_capability(797)
set_process_capability: cap_set_proc failed: Operation not permitted
[2007/12/20 20:14:26, 0] lib/util_tdb.c:tdb_log(662)
tdb(/var/cache/samba/printing/pdf-gen.tdb): remove_from_freelist: not on list at off=20728
[2007/12/20 20:14:26, 0] lib/util_tdb.c:tdb_log(662)
tdb(/var/cache/samba/printing/pdf-gen.tdb): tdb_free: left free failed at 20728
and printing to the Linux box connected HP5740 gives this error:
[2007/12/20 20:15:32, 0] printing/print_cups.c:cups_job_submit(656)
Unable to print file to HPDesJet5740 - client-error-document-format-not-supported
(in the client.log)