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Indeed, polished UI elements tend to be done in Photoshop, while those for AI tend to be for wireframing. Here are a few links:

http://developer.yahoo.com/ypatterns/about/stencils/

http://www.webalys.com/design-interface-application-framewor...

Note that not all Photoshop is raster, however; shape layers can be used to create vector elements. For example, the following is vector despite being PSD:

http://designmodo.com/polaris-free/