There are a number of different factors that makes Poser figures easy to spot, it's the mesh, the topology and the layout of muscles etc.
Also they're quite common and they got alot of exposure on Daz3d.com and Renderosity and other reseller sites.
And also that just about everyone has once in their life stumbled upon a Poser model.
But don't take crap for using Poser models in your work, alot of people dislike that since it's premade models. That's just silly argumentation and it should be ignored. Art is art no matter how you choose to create it and the tools you choose to create it with.
/ Max