I can't tell you how many startups I've run across who are deperately looking for programmers who know [insert cutting edge/obscure technology here].

Because they can't find anyone they like, they are stuck making bad/deperate hires or with the prospect of rebuilding the codebase. The list of technologies that are tough to hire for vary by market. In my midwest market, Rails still falls in that group and I know a ton of companies that can't find the talent they need. Do your best to stick with the technology stack that has the largest candidate pool in your market.