0.8.0 doesn't require an upgrade. The special thing about 0.8.1 is that it has special time-limited block validation restrictions that are intended to enable the pool operators to upgrade. After May 15th, 0.8.1 behaves the same as 0.8.0. More than 51% of the mining hash power is applying the 0.8.1 restrictions to make sure no bdb-busters get into the chain.
As I understand, it was 0.8.0 client generated backward incompatible block last week, obviously they need to upgrade. Can you elaborate on that "time-limited block validation restriction"?
*All* block generators can be configured to generate a block that will break 0.7.x and earlier. The problem is a valid block that passes all the bitcoin rules, but fails in a 3rd party library in an unpredictable way.
So 0.8.1 has a special set of code..
If (current_time < may 15th), reject otherwise-valid blocks that might close to breaking 0.7.x and earlier. The majority of the hashpower runs this patch so that means that should somebody else generate a bdb-breaker block, most of the hashpower will orphan it and the network will reconverge on a chain that everyone can deal with.
The wildcard is all these heavy hitting mining operations with asics etc. It is not out of the bounds of possibility that the big pools right now might not be able to scrape together a 51% majority for much longer. New hashpower is being turned on at a great rate and not all is going into recognizable pools.
Think about it.. if you had 50+ avalons in a room trying to pay themselves off, why not run your own bitcoind and get 100% of the revenue and 100% of the fees? Folks like that probably aren't interested in an some future emergency workaround that involves them discarding valid blocks.. if they're even reachable at all.