Excerpts from Wael M. Nasreddine's message of Tue Jan 25 05:34:32 -0500 2011:
> Just add the inbox label to it ( move the cursor on the thread and press l,
> l for like! and type inbox)
> 
> This should work

Frustratingly, it doesn't. You can't add 'reserved' labels using 'l'.

I have no idea why that's the case, though.