There are 2 trusted domains:
DomainA.com (WorkgroupA, Administrator userA/passwordA)
DomainB.com (WorkgroupB, Administrator userB/passwordB)

Linux client has smb.conf and krb5.conf files with DomainA information
configured correctly.

#net rpc join  -U userA%passwordA
Joined domain WorkgroupA.
#net rpc join  -U WorkgroupB/userB%passwordB
Joined domain WorkgroupA.
#net "rpc join  -W WorkgroupB -U userB%passwordB
Joined domain WorkgroupA.

#net ads join -U userA%passwordA
Using short domain name -- WorkgroupA
Joined 'Client' to realm 'DomainA.com'
#net ads join -U WorkgroupB/userB%passwordB
[2006/05/06 03:58:45, 0] libads/kerberos.c:ads_kinit_password(145)
  kerberos_kinit_password WorkgroupB/userB@DomainA.com failed: Client not found in Kerberos database
[2006/05/06 03:58:45, 0] utils/net_ads.c:ads_startup(191)
  ads_connect: Client not found in Kerberos database
#net ads join -W WorkgroupB -U userB%passwordB
[2006/05/06 04:01:11, 0] libads/kerberos.c:ads_kinit_password(145)
  kerberos_kinit_password userB@DomainA.com failed: Client not found in Kerberos database
[2006/05/06 04:01:11, 0] utils/net_ads.c:ads_startup(191)
  ads_connect: Client not found in Kerberos database

#net -V
Version 3.0.14a
Client runs MIT Kerberos library
Domains are Win2K3 SP1 servers