The documentation at
http://roundup.sourceforge.net/doc-0.7/customizing.html#the-context-variable
suggests that the 'context' variable is always defined
and assumes three different kinds of values (None, a
class, or an instance), depending on the request.
In reality instead of being 'None', the context
variable doesn't exist if the tracker's 'home' page is
accessed, so instead of 'tal:condition="context"'
the user has to use 'tal:condition="context | nothing"'
to avoid an exception.
Just a clarification...

Logged In: YES 
user_id=6405

Hrm, that's odd. The code clearly has context being defaulted 
to None. I'll have to poke some more. 

Logged In: YES 
user_id=6405

Ehem. Richard looks at maint-0-7 code instead of HEAD, and 
fixes the bug. Sorry!