There are a few bits of interesting information in the PDF manual that bear noting. Although none are actual spoilers, rather than posting them here on the front page, we'll just link to them; so to see the list click "read more" below.

• The manual notes a credit to Havok.com, indicating that Halo 2 does, indeed, use the Havok physics engine to some extent.
• The voice credits list Julie Benz as Miranda Keyes. Wife? Daughter? Sister?
• Several prophets are mentioned: Hamilton Camp as the Prophet of Mercy, Robin Atkin Downes (any relation to Steve?) as the Prophet of Regret, a credit that was leaked a long time ago at IMDB; and Michael Wincott as the Prophet of Truth.
• Another interesting credit is Tim Dadabo as 343 Guilty Spark, indicating that he'll be back in the game, and John Michael Higgins as 2401 Penitent Tangent, which sounds to me like another Forerunner AI, so it seems reasonable to expect that the story will be taking us to another, possibly intact, Halo installation.
• Other characters of note: Dee Baker as Gravemind, Keith David as Arbiter, Miguel Ferrer as Heretic Leader, indicating some dissension in the Covenant ranks (another well-used Bungie theme) Ron Perlman as Lord Hood (Robin Hood-- perhaps the pirates near Eridanus survived?) and Kevin Michael Richardson as Tartarus, the Brute.

As these are in the manual available online at Bungie.net as well as open to any player before they start playing the campaign, none of these facts should be considered spoilers, per se.