Microsoft caused a lot of excitement when they announced Windows Phone 8, but also a good deal of disappointment when it became clear that the new OS won't be coming to the current crop of Windows Phone devices.
There's a reason for that - the two OSes may look similar on the outside, but they are vastly different on the inside. The new core that enables so many cool features (multiple CPU cores, better graphics) is shared with Windows 8 RT (based on Windows NT) and not Windows Phone 7 (based on Windows CE, same as Windows Mobile).
Microsoft’s senior marketing manager for Windows Phone, Greg Sullivan, says that it's not impossible to port WP8 on older devices, but the cost of doing that would be very high and the benefit very little - WP8 enables multi-core support, higher resolution screens, NFC, microSD card support and so on, none of which will make a difference on the legacy hardware.
What they are getting (as part of the Windows Phone 7.8 update) is the new start screen - it's the most noticeable change in the OS and will actually benefit the current Windows Phone smartphones.
Users who move to the new version of the OS will get to keep using their Windows Phone 7 apps as those will work on WP8. There's no backwards compatibility for the newly developed WP8 apps though and there's nothing that can be done about it - for example, the Adreno 205 just won't cut it for games intended for a Snapdragon S4 (which packs an Adreno 225 or 305).