Using gmyth 0.4, and the mythtvsrc from the current CVS (ie. the rewritten one).

$ gst-launch playbin uri=myth://192.168.1.5/1006_20070920210000.mpg
Setting pipeline to PAUSED ...
ERROR: Pipeline doesn't want to pause.
ERROR: from element /playbin0/source: Could not initialise supporting library.
Additional debug info:
gstmythtvsrc.c(677): gst_mythtv_src_start (): /playbin0/source:
Could not begin request sent to MythTV server (0, myth://192.168.1.5/1006_20070920210000.mpg)
Setting pipeline to NULL ...

GThread-ERROR **: file gthread-posix.c: line 171 (): error 'Device or resource busy' during 'pthread_mutex_destroy ((pthread_mutex_t *) mutex)'
aborting...
Aborted

Passing the port (6543 should be the default when not mentioned, as it's the default for mythtv installs), fixes the problem. Ie:
$ gst-launch playbin uri=myth://192.168.1.5:6543/1006_20070920210000.mpg

Let me know if you can't reproduce and need a backtrace.