If we get away from the baggage of the word "shortage", it is a true statement that the hiring market currently favors those who are selling talent as opposed to those who wish to purchase talent, across a wide variety of first-world metropolitan areas, in many fields of software engineering, including but not limited to fairly generic web and/or application development.
Digging a bit deeper - I do think there's a pretty universal shortage of high-quality developers in the software world. Understand your CS fundamentals + current technologies and have decent social skills and you will never lack for work. I think that Google and Facebook have gobbled up the vast majority of these developers, because they are willing to pay higher wages than most startups can. That's the way economics works: the people who can pay the most get first crack at the supply.
I think that's the real sticking point - quality developers are going where they'll get paid what they're worth, and in most cases that's not startups on shoestring budgets that could go under at any minute. It helps to keep in mind that quality developers are often older, with families, who want to find somewhere they can be well-compensated for putting in a solid day's work before going home in time for dinner. This doesn't seem to jive with what a lot of startups expect of their employees.
By the way, I don't mean to put all startups under this generalization: this purely based on anecdotal evidence from myself and people I know.
In my own startups I've found the developers I know love the idea of working for a startup. But when it comes down to it, they are risk averse and value job security and cash compensation over the freedom and equity that a startup offers.
I don't know what "for startups" has to do with it, but if it matters, I work for a 90-person company that calls itself a "mature startup." :)