Bearing in mind that this is rank speculation, Apple does what Apple does (I remember how surprised I was when the original iMac was announced), here's my thoughts on the next iteration iMac:
I'll go with "3. Later than March 17th."
Although the iMac has a similar foundation as the laptop line, why impact laptop sales by making it compete out the door with the iMac for parts? Apple makes lots of money on laptops, so they will be first in line. The make less money on iMacs, but iMacs drive other things (iPods/iPhones/market penetration, etc.). Also, the real key to the iMac is that it is a consumer model. "Consumer" for Apple has long had a "student" component. Think of all those kids going to college in the fall for the first time. They need a new computer. They want a hot new iMac that can fit in a dorm. Look for the iMac in the June/July timeframe so that production is really ramped up by August for all those students. They are good for 250,000 in sales, maybe more if Apple hits the crease right.
What hardware will be updated?
1. Processor - intel's 45nm...that's seems like a given. Any chance of a quad-core (montevina) option? No Montevina until 2009, the following iMac upgrade. Why confuse things or become dependent on potentially slow moving parts in the fall?
2. Graphics Card - Minor move upwards, but nothing really robust. That just isn't the Apple way. Look for solid, but not spectacular, something that we already see plenty of in the marketplace....
3. Matte screen - no, this is a consumer model. They won't burden it with too much.
4, & 5 - Apple always goes short on memory, it stays at 1GB. Look for larger hard drives, but only because they are cheaper to acquire (see the Time Capsule for a hint about that)
6. Price Drop - Would be nice to see the $999 iMac again. Maybe for a very low-end model, but now that they have abandoned 17" monitors, I am not sure I see it happening. $1099 is the new $999, just like 40 is the new 30....
7 & 8 - no other improvements. Incremental upgrade. They might even stick with the Aluminum iMac for another iteration in 2009 as well.
I desire to replace my G5 2GHz, but I think 2009 is the year for me. It still does everything I want it to and it doesn't seem slow yet, like my G3 did before it.