The biggest problem with ASIC miners that I see is that they do not ship until "a few months". With the mining difficulty growing exponentially [1], the hardware you buy now will be relatively obsolete in a few months. For example by buying the latest 600GH/s from Butterfly labs[2] at 4500$, you'd break even in about a month according to the acula mining calculator[3].

The problem is that your card will arrive in January. In 2 months, if the bitcoin difficulty keeps growning at a steady pace (which is improbably, it probably keep growing exponentially), the break-even will be closer to 2 months. While you will probably keep making a few hundred of dollars after the break-even point, your chances to "double up" your initial 4500$ investments are close to zero. I have no idea about the profit marging ButterflyLabs make on those cards, but I'd bet it's larger than "a few hundred of dollars". That's why the mining gear business is probably more profitable than the mining business itself.

Of course this doesn't stop Butterfly Labs from having large farms of those cards...

[1] http://bitcoindifficulty.com/

[2] http://www.butterflylabs.com/monarch/

[3] http://www.alcula.com/calculators/finance/bitcoin-mining/