The last two times Intel launch a new CPU architecture (Sandy Bridge and Ivy Bridge), the new CPUs were available at very good prices for the first few days and then the prices inflated a little due to the stock drop and strong demand. I expect the situation to be the same for Haswell's launch. Intel will no doubt have an Haswell Core i5 at a similar price as the Ivy Bridge i5 3570K, but with slightly higher performances and lower TDP. Intel is supposed to have been shipping Haswell CPU for the past few weeks to big retailers, sot there will be a good selection of CPU in stock at launch.
Regarding the motherboards, sure, there won't be as wide a choice at launch as there is currently for Ivy Bridge. But there will be several for sure. I would even be surprised not to see those appear on my price lists in the previous two or three weeks before the official launch. I remember having ordered an LGA1155 motherboard (a P67 in that time) before Sandy Bridge arrived. I would bet it will be the same this time. With the catastrophic sales numbers for the PC market that have been made public last week, be sure the motherboard makers won't screw Haswell's launch. The motherboards will be there.