This is every rumor the I have heard so far, and my opinion on it:
1. The 4.3L V-6 is going to be replaced. I think this is going to happen in the next couple of years, and the likely replacement will be a version of the new 3.9L V-6 that is used in the Impala. The Vortec 4200 in-line 6 is too costly to be a base engine.
2. The 8.1L will return. I don't see this happening for a couple of reasons. First off, remember that the 8.1L was dropped from the '07 Classics due to lack of sales. Second, the Tonawanda engine plant that builds the 8.1L is slated to be completely renovated to produce a new 4 cam high performance V-8 for Cadillac in a couple of years. I doubt 8.1L usage in medium duty trucks, industrial and marine applications will be enough for GM Powertrain to justify keeping it around. But- this is not to say that there won't be anything larger than the 6.0L for the HD's in the future.....
3. GMT-900HD's are going to get a new chassis in '09. Yes, it is true that the 900HD's use the 800HD chassis with some minor modifications. This rumor will not go away, but nonetheless I have not heard or read anything that supports it. The rumor is that the new 4X4 chassis will feature a 9.5 AAM straight front axle like Dodge uses on the Ram, the towing capacity will surpass Ford and Dodge, and the only reason it isn't out yet is because GM wanted to introduce the 900HD's this year, and the new chassis was not ready. It sounds good, but there is just no proof.
4. There will be a diesel for the light duty 900 trucks and SUV's. Looks like this one is true, a new V-8 diesel for these trucks is widely reported to be under development.
5. All the 900's will feature a 6 speed automatic. This too looks like it will happen. The new family of Hydramatic 6 speeds, the 6L70, 6L80, and 6L90 are just around the corner (in fact appearing now in gas engined 900HD's). This is a case of getting the truck to market before all the components were ready.
6. SS 427 Silverado. I think this one is likely! Many rumors about exactly what it will be, but I for one am hoping for a regular cab short bed.