Very recently I followed a question at activedir.org (very interesting mailing list!) concerning whether external trusts support Kerberos. (topic @ activedir.org)
Microsoft isn't always as clear about it, but the following articles does state it: Kerberos is only possible when a forest trust is created: http://technet.microsoft.com/nl-be/library/bb727065(en-us).aspx
And some other references:
Access to resources between domains that are connected by an external trust requires Pre-Windows 2000 Compatibility. Because external trusts only support NTLM authentication, queries to a directory in a different forest are always handled as anonymous access.
If you use an external trust NTLM is used as authentication protocol, no Kerberos involved. Kerberos authentication only works if the trust type is “Forest Trust”.
External trusts only support Integrated Windows authentication (formerly called NTLM) for the user access.