There are two issues around blocking in SP4. One's a real bug, the other isn't.

The first one is a bug in the lock manager that manifests during DBCC INDEXDEFRAG. The lock manager erroneously holds onto NULL mode locks when INDEXDEFRAG drops its page locks, eventually resulting in escalation to a table lock. This will block other queries. There is a hotfix available. Details can be found at http://support.microsoft.com/default.aspx/kb/907250

The other issue is an enhancement to the blocking info displayed. It may look like a SPID is blocking itself but this is in fact normal during synchronous IO. More info at http://support.microsoft.com/default.aspx/kb/906344

AFAIK there are no issues that would cause the problem you describe - recommend you call PSS to assist you.

Thanks