The custom filter is a good explanation. It's the only one I can think of actually. Great firewall appart...

I hope this is not the case. Filtering transactions is not good for bitcoin. It is also not good for ASICminer shareholders.
I personally have had several transactions that were not initially confirmed when ASICminer found a block. I had to wait for the next block to be found by someone other than ASICminer to get an initial confirmation. I included a .0005 fee on each of those transactions, which I thought was enough to get my transaction included.
I doubt it's this - I would say it's more likely that connections from within China are able to get to fewer connections outside of China. It's not just the government firewall, peering is just not straightforward when it comes to this country.