A good article on PTSD nightmares in which prazosin is used.
Update: Clonidine and guanfacine are also used and, in error, I had substituted 'gunfacine' where 'prazosin' was correct in the original post. All are alpha2 agonists (see Aug 26, 2009 post below). Clonidine will suffice for a moderate or less nightmare problem; guanfacine has a longer half life and may be preferable in late night dreams. Guanfacine is reported in a child related letter I believe.