Warner Brothers will release the comic-within-a-film Tales of the Black Freighter on DVD, five days after Watchmen release date. "The DVD will also include a documentary-style film called Under the Hood that will delve into the characters? backstories." Source.
According to SCI FI WIre, "Eventually, Tales of the Black Freighter will be followed about four months later by the DVD release of Watchmen itself and then an "ultimate" edition in which the two are edited together into one megamovie, the newspaper reported."
Watchmen is set for release on March 6, 2009.
Source: SCI FI Wire.