I'm not an accountant, but I am a financial professional. I read a quick summary on the FASB website and I'll give my first impression of what it's saying:
It's possible to own things which are difficult to assign a dollar price to -- say, for instance, the right to a percentage share of the payments on a group of mortgages, or drilling rights on unexplored property. Still, you need some way of managing this ownership on your balance sheet.
It's possible that you might decide to refine the way you are accounting for this possesion in order to get more accurate results. Supposing that this revaluation causes some sort of loss, this is going to affect your balance sheets going all the way back to whenever you acquired the property. Previously, the rule was that you only had to change your most recent earnings report to compensate for this issue.
The new rule, it seems, is that any accounting change of this sort requires you to go back over all your previous financial statements and revise them to take the new accounting procedures into consideration, retroactively affecting your earnings from previous years.
If I were you I'd double check my explanation against the FASB's a little more carefully though -- I'm not claiming to be an expert on the subject, or even to have read the whole thing.
