I think the exchange rate will keep getting higher as the difficulty increases.

My reasoning: As the difficulty increases, so too does the security of the Bitcoin network. It also increases the value of one Bitcoin relative to one gigahash. Now, looking at it this way, some amount of money will give you some amount of computational power. The value of a gigahash in Bitcoin will change as the difficulty increases, but the value of a gigahash in US dollars or in Euros is much less malleable. Eventually, the value of BTC will go up relative to USD or EUR, for this very reason.

So, a 25 ghash/s ASIC will net you decreasing amounts of BTC over time, but it will probably give you the value of 25 ghash/s in EUR or USD, which isn't a trivial amount and decreases much more slowly.

So: The bad news is that your 25 ghash/s will give you less BTC than you are expecting. The good news is that those BTC will be worth more in EUR or USD than you are expecting.