Very-low-priority transactions will get dropped by peers, because they look like "spam" transactions and network bandwidth is not free.
+1
This is a problem I hope they find a solution to, very annoying.
Just to be clear though, this does not mean that the transaction will not ever get processed.
Though some peers may not relay a very-low-priority transaction the client will continue trying to resend that transaction to its peers.
If the problem is the transaction isn't getting relayed, the client can even be directed to connect to a specific node that relays regardless:
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http://en.bitcoin.it/wiki/Free_transaction_relay_policy (though I don't know if that will cause the transaction to be included in a block any sooner.)
In my case the transaction was created by the Andriod app "Bitcoin Wallet". I have no way of pointing it to specific peers.
Anyway, as far as I know in the moment all nodes run the same software, because except of bitconj there is only one implementation. So why should some of peers relay the transaction while others do not when they are all running the same software and following the same rules?