Developers please note:
https://bitcointalk.org/index.php?topic=152030.0And now a more elaborate explanation:
0.7 and older nodes use BDB for storing the blockchain databases. It seems this database has a limit on the size of the modification it can make atomically to the database. With the larger blocks of the past days, it seems to have triggered the limit. The result is that 0.7 (by default, it can be tweaked manually) will not accept "too large" blocks (we don't yet know what exactly causes it, but it is very likely caused by many transactions in the block). Specifically, block
000000000000015c50b165fcdd33556f8b44800c5298943ac70b112df480c023 (height=225430) with >1700 transactions.
However. 0.8 (which uses a different database system) has no such limit, and happily accepts the block. As the majority of the hash power was on 0.8, the longest chain ended up using this block, which is not accepted by older nodes.
The solution is to (for now) go back to the old chain, which has block 00000000000001c108384350f74090433e7fcf79a606b8e797f065b130575932 at height 225430.
