I think a lot of people must know who she is. I'm only a recent and very casual skimmer of her blog, but even I see there are plenty of clues as to her identity. Apparently Ms. Snark is a woman. She's in New York. She works alone. She seems to be a fiction specialist. Except for that she claims to love Pynchon apparently she favors mainstream commercial fiction. I don't see much evidence that she handles any genre stuff. My conjecture is that her taste for Pynchon is personal, and not a model for her client list, nor does she represent him. She seems to dislike controversial or disturbing works. She won't consider long novels. She's very businesslike and bottom line oriented in her outlook. She has a taste for mysteries. Her ostensible infatuation for George Clooney as well as reverence for Pynchon suggests to me someone past 40. This is what I've picked up skimming only her blog for a few weeks. I may be wrong on my facts here and there, but I'm sure those who have read her longer and more closely than I have could identify other clues.
The stuff about the poodle and the gin seem to me likey props. She may have a poodle someplace and she may occasionally enjoy a martini, but I doubt it's like she says. But stuff like her gender, her favorite books, her predilections as an agent, where she works, her ideal man, I doubt she'd lie about just to preserve her identity. The fact that her identity is ostensibly unknown and rarely speculated upon seems to me to say more about culture and norms in the publishing industry (I don't claim to know what they are) than how hard it would be to unmask her if someone really cared to do it.