The last across-the-board price increase by WDW was in September '02. I remember that vividly because it was about 3 weeks before we visited.
Earlier in '03 there was an increase in the consumer end price due to a Florida sales tax increase. There was also a mid-year increase in gate prices only (pre-purchase prices did not go up).
I wouldn't stress too much about the possibility of a price increase. When WDW does increase their prices, it will be all over web sites like this. The good news is that third-party ticket dealers like Ticketmania or Florida Orlando Tickets will NOT immediately increase their prices. They will maintain old prices until they exhaust their supply of passes that were purchased at the lower prices. So realistically you'll have a couple of weeks to get your passes at a lower price even after WDW increases their rates.
Of course this doesn't apply to passes that can only be purchased from WDW (single day, annual passes, etc.) But any of the multi-day hopper passes can easily be obtained from reputable dealers at good prices after an increase.