This is just my opinion
I think most people who own an ipad will keep their's excluding the people who will upgrade no matter what apple does. The next ipad cannot be that much better as it would go up in price and create more fragmentation. The 500 dollar price point is key and apple would most likely (just everything else they sell) make a minor changes.
It might have a new processor, more memory, cameras, and barly better display at same resolution. Now to some this sounds like a lot but if you own an ipad you know that it already runs apps just fine. A much better ipad would need specific apps to shine thus creating more fragmentation (along with super cheezy app names like "super duper HD")
Ipad are luxury toys because nobody needs one, I'm an IT guy heavy into technology and even I could go without it. Ipad is convienent so there is no demand really just hype. It is the first and best tablet of it's kind but honestly if apple wants the ipad to be better it will be by software not hardware.
I know many people are waiting to buy the ipad 2 instead of the first one but your just wasting time and money as you could be enjoying it now, you could get it pretty cheap as people are selling them for like 350 on CL, and even if you did want to upgrade you could just sell it and you will get the same price as a person that owned it since when it came out and can use that towards the ipad price. If you buy cheap you can always sell high you have nothing to lose. Of all of the ipads I own I can't tell the difference in any of them all look the same work the same.
Apple right now has no competition in tablet markets and won't for another year at least. Android, WebOS, RIM all don't have the apps. Knowing apple they will just keep it simple, minor and stay at the same price point, phase out current gen, then bring in the new and sell another 15 million again maybe.
To me the ipad is a cool 300 dollar device and nothing more, higher hard drive space and 3g are both totally worthless as you don't need all that storage at the same time and you should own a phone buy now that you can tether for free. Only maybe if you too lasy to sync you ipad and pick what you want to go on there but 16gb is more than enough. Now if you feel having 5gb in pictures is important or 5 gb in music is important or 6gb in movies then maybe the higher storage is for you but mostly all of that stuff will just sit on your ipad and waste space as you will barly use it.