Well that's exactly what ASICs are going to do. They are going to atleast (Probably) double the difficulty of mining BitCoins. The problem with this is going to be that unless you have an ASIC or a computer that can mine at least 1GH/s then you're not going to want to mine anymore because the difficulty is so high. This will ultimately cause the difficulty to drop back down. Eventually we will reach a point that's about 145-160% the difficulty now with the release of ASIC Miners.

To answer the question if these GPU miners are becoming obsolete, For the most part yet, for BitCoins at least, a computer that pulls under 1GH/s is not going to make much in the lines of mining and the electricity cost is going to probably be more than your return will be.

LiteCoins are the future of GPU Miners.
BitCoins are the future of ASIC Miners.