How is it considered by hiring managers? How long one should stay at one place to not be classified as a hopper? Is it a big deal to hop in our (the tech) industry for engineers/PMs? Does it makes a difference whether the hopper apply at a startup or at a big company?
(also I consider a hopper who's efficient and who's seeking challenges, who unwilling to settle rather the underperformer who keeps leaving before getting fired...)
People define job hopping different ways, and the tenure matters. I once had a client (financial, big) that wouldn't hire anyone that wasn't at their current job for 7+ years. They abandoned that policy around 2006, as they found it impossible to hire, and they also found that the types of candidates that met that qualification often weren't that attractive. In other words, they found that if you were at your company for 10+ years, it could be because you are not in high demand by others.
In tech, moving jobs is expected and probably recommended from a marketability and employability perspective, as long as you make smart moves and don't just leave every time you get bored or passed over for a promotion.
Someone who has a pattern of staying perhaps 3 years with companies and then leaving will often be viewed as very attractive, whereas someone who has several 6-8 month stints over the course of a few years will often get a negative view. EDIT: Forgot that I wrote an article on job hopping for tech pros earlier this year http://jobtipsforgeeks.com/2013/07/25/hop/