users of pre .10 cannot generate a valid block until they upgrade.
Precisely. I've tried to write a simpler explanation but I can't get it out, even though I understand the problem here.
A more complicated answer: Generated coins with pre .10 clients will be summarily rejected because they are trying to incorporate the transactions that caused the problem in the first place. This is the cause of the forking going on right now, and it is a good test for the strength of the network to root out bad blocks.
Simple advise: Upgrade to the latest version of the client if possible. Get the word out if you can and note this is important if you intend to generate new Bitcoins. To the best of my knowledge, the older clients will work fine for simply performing transactions (sending or receiving coins) but even there it would be a good idea to upgrade if possible to "strengthen" the network overall and not let those transactions get caught up into bad chains.