Many interpretations, including letterbox being screen format (eg in DVD), 24 dc being 24 December, 24 being the number of letters in domain, etc. Flickr has a photo titled exactly the same as clue - http://www.flickr.com/photos/73728080@N00/2067705390/
Thanksgiving related. The 'V' sign yet again. But the photo title could be a coincidence.
It is also now being pointed out (After clues relating 'Fast' coming fast!) that 24-dc1 could mean X-43A (24=x, d=4,c=3,1=A) another hint about 'FAST' X-43A screamjet did a record speed too.
Another (more probable solution) - Added on 28 july (ThanksWhirlpool)
in' is a typical cryptic term to indicate that the word could be inside, thus 'URL' can be seen in 'in yoUR Letterbox'. 24-DC1 may us something about the url or the domain we are after. 24 could be the number of letters in the url /domain. DC1 could be anything e.g. DC = Dot.com which we already know