I have had the same problem burning dvd video files with button grayed
out,I think I have found the solution,here it is:

I use Nero version 6608

put blank dvd in dvd-rom
clik Nero StartSmart icon on your desktop
select dvd data,
drop dvd files there clik next.
burn button will not be grayed out
don't burn dvd yet.On bottom tool bar clik
NERO ICON when wizard pops up clik cancel
go to nero wizard CLICK FILE NEW
click new dvd-video like usual and the burn button won't be grayed out
and you should be able to burn your dvd
I hope this helps if it doesn't work leave dvd blank in dvd writer-rom
reboot computer and repeat steps as above

(in reply to echriste)