Does anyone know why I cannot use my 8700 series battery in my 8800? I get a slashed battery icon on the screen. I even tried an extended 8700 battery with no success. I also noticed that the new battery is green in color. Can anyone explain? Is it blackberry way of making more $$ and forcing us to upgrade?

Can anyone explain?
Quite simply, they're different batteries and not cross-compatible with eachother. The "blue" C-S2 battery won't work in a device that requires a "green" C-X2 battery, and vice versa.

Is it blackberry way of making more $$ and forcing us to upgrade?
Forcing you to upgrade? Nonsense. Your 8800 didn't come with a battery of its own?

Hmm, well they are private business with a goal of making money.

And how is your switching batteries between the two devices forcing you to upgrade?

I have a extended battery from my 8700, now it doesn't work in my 8800. Thus forcing me to upgrade my extended battery when it comes available.

Oh, please!
No one is FORCING you to buy an extended battery. That's your choice.
You chose to get an 8800. If you assumed that your 8700 battery would work for it, then that's your own mistake.

Using an extended battery is a choice you are making. Your "victim" complex is showing.