sorry might sound quite "stupid" but as the difficulty scales go up (meaning less bitcoins are generated per mh/s) would it not cause by default, an increase of value of BTC as they become more "rare" or "harder" to "produce"?
Only if the rate that people buy BTC keeps up with the rate BTC are generated.
If there is more demand than ~7200 new BTC/day (plus what old miners are selling from their stashes, so maybe ~8000 BTC/day for a while still) price will go up, if there is less, price will go down.
Difficulty wil FOLLOW price (as it might get unprofitable to mine at some points) but not vice versa - a merchant will nearly ALWAYS see ~7000-7500 BTC per day generated regardless of difficulty (and this is the only thing that drives price) unless a lot of miners quit at the same time.