MSDN documentation has revealed that a major upcoming feature to the Windows Phone platform - in-app purchases - will be exclusive to Windows Phone 8. Microsoft advises developers that you can only add in-app purchases to Windows Phone 8 applications, indicating that legacy devices updated to Windows Phone 7.8 will not receive this functionality.
This also seems to suggest that the Wallet Hub announced alongside Windows Phone 8 will not make its way to Windows Phone 7.8, as the Wallet Hub will be in control of in-app purchases along with other things such as NFC payments, deals, frequent flyer information and receipts. So far only the new Start screen is confirmed for WP7.8, and it looks like most of the WP8 features will not make it to original Windows Phones.
Microsoft is said to be currently finalizing Windows Phone 8, with a release still slated for this holiday season. Expect announcements of new Windows Phone 8 devices as early as September.