[Dr. Paris]

I'd put money on March/April '09. By all accounts, that's the release date.

Things that look almost certain:
Doctrines are gone
Platoon Drill that will allow you to shoot through your own men
Customizable platoon options (replacement for doctrines)
New Plastic Russ kit which can make all variants
New HQ "Commisar Lord" (seen pics of him online)
Plastic Stormies
Plastic Valkyrie

Things that are likely, but still iffy:
Plastic Greatcoats (these have been talked about for awhile)
New Russ Varient (you can see a picture in the 5th Ed book)
3 Leman Russ squadron (Unknown if it's 0-1, or a regular HS choice)

Things I personally think we'll see:
IG lineman drop to 5 pts/model
A host of special characters, some which will legionize the army (same as Orks and new SM)
Plastic Rough Riders

I think getting new storm troops, great coats, Leman Russ, Valkyrie, Commisar Lord, Special character models, and rough riders sounds like a decent codex release.