As BTC prices currently don't keep up with its difficulty rise, it can sometimes make sense to make a mining switch.
Remember, price controls difficulty. Not the other way around. And everything is a complete mess right now, and will continue to be so for a least a few more months.
I would concur that price and difficulty could be considered correlative, but there are other factors involved like media exposure, for esample. The more BTC makes the news, the more people get interested and believe that they could see some benefit from their home PC's.... Of those, some will quit and others will invest in better hardware. It is the latter that increase the Network hashrate along with current miners that need more/better hardware to continue to remain profitable, which in turn causes the difficulty to increase every 2016 blocks (which will be, as I write this, in 655 more blocks at an average rate of ~ 8.25 blocks per hour). http://bitcoincharts.com/bitcoin/
On the other hand, difficulty is determined by expected block time versus actual block time in a very fun formula explained quite well here: http://bitcoin.stackexchange.com/questions/5838/how-is-difficulty-calculated/5840
To find out what the current Network hash rate is: found/expected*diff*2^32/600. You can find input values for this formula from the previous links.
So, yes, for those of us who can no longer compete even by being with one of the large pools, switching to other coins is proving to be more sustainably profitable over time.... until those Scrypt ASICs hit the market..... then the race starts all over again. http://majesti.co/cryptonerd/new-scrypt-asic-miner-announced/
