IMDB has been brought up a few times in various discissions:

19 April 2007
Wikipedia talk:WikiProject Films/Style guidelines/Archive 1#IMDB User Ratings
don't use the user ratings
18 July 2007
Wikipedia talk:WikiProject Films/Archive 14#IMDb as Valid Reference Material
resulting in an failed proposal here.
7 January 2008
Wikipedia:Reliable sources/Noticeboard/Archive 5#IMDB
IMDB may use information from Wikipedia in user submitted areas
16 April 2008
Wikipedia:Reliable sources/Noticeboard/Archive 11#IMDB Pro
IMDB Pro is no more or less reliable than the normal site.
18 July 2007
Wikipedia talk:WikiProject Films/Archive 14#IMDb as Valid Reference Material
resulting in an failed proposal here.
25 July 2008
Wikipedia:Reliable sources/Noticeboard/Archive 17#Is IMDb a reliable source?
not as reliable as the new york times.
27 September 2008
Wikipedia:Reliable sources/Noticeboard/Archive 22#IMDb
2 October 2008
Wikipedia:Reliable sources/Noticeboard/Archive 20#Are IMDB and personal websites reliable sources
17 November 2008
Wikipedia:Reliable sources/Noticeboard/Archive 24#Is IMDb an unreliable source?
18 November 2008
Wikipedia:Reliable sources/Noticeboard/Archive 23#IMDB.com for plot synopsis
9 February 2009
Wikipedia:Reliable sources/Noticeboard/Archive 28#IMDb for BLP info?
23 June 2009
Wikipedia:Reliable sources/Noticeboard/Archive 37#IMDB
26 July 2009
Wikipedia:Reliable sources/Noticeboard/Archive 40#IMDB, again
17 October 2009
Wikipedia:Reliable sources/Noticeboard/Archive 47#IMDB?
25 November 2009
Wikipedia:Reliable sources/Noticeboard/Archive 50#TV.com and IMDB
6 March 2010
Wikipedia:Reliable sources/Noticeboard/Archive 58#IMDB as a source; List of documentary films

Perhaps the most telling is the IMDb FAQ, which states that they use on-screen credits. I take the position that the credits section is reliable, and a good secondary source for that information.