Enabling threads seems to change the event loop behaviour. On 8.5a6 unthreaded the event loop runs ieven if there are no event sources: 'vwait forever' in a freshly launched tclsh just "hangs" indefinitely.
In an unthreaded build we see the well known
% vwait forever
can't wait for variable "forever": would wait forever
This is on Linux/i686, kernel 2.6.20