I would've thought that at the point bitcoins could no longer be mined they would become increasingly rare and therefore be worth more. It would be a bit like mining gold, if gold mining stopped tomorrow the gold price would increase.
On the down side, there is a couple of things that could affect bitcoins such as whether they remain popular in the future and whether govts around the world end up banning them. Technically in Australia the only legal tender is the $AUD.
