Can a fail whale float?
Most web "phenomena" last a few years. They grow and grow, become the darling of the online press - then the wind changes and all their users desert them (sometimes helped by unpopular changes, managerial incompetence or the next "big thing" being bigger and thingier than they are). Twitter has had a good run and round about now you'd expect it to start sliding as the next generation of internet users dismiss Twitter as being "for old people".
So why go for an IPO? Possibly to staunch the slide. Buy up the competiton, or expand by aquisiton - or maybe just to cash out while there's still some money in Twitter as a going concern.
Either way, given the current climate there's no good time to float (or sink), but on the presumption that the financial situation won't be getting any better for some time, and that Twitter's shelf-life could be coming to the end, this would seem like a good time for them. Whether it's a good time for investors? No-one can say.