Update 2016-01-14: Added noise floor of Symmetricom 5115A:
The question of using a general purpose Spectrum Analyzer vs. a dedicated phase noise test set for measuring phase noise came up on the EEVBlog forum. Here are two noise floors for 10 MHz phase noise measurements measured recently. One with a R&S FSW8 spectrum analyzer and the other with a Microsemi 3120A phase noise probe. The 3120A noise floor improves by maybe 10 dB in a longer duration measurement (e.g. hours instead of minutes). On the other hand the SA can measure phase noise out to at least 1MHz offset frequency and at any (I think?) carrier frequency up to several GHz.