A previous post answering the hypothetical "say I write like David Sedaris, how do I go about pitching an agent" roiled up some interesting comments.
Let me be address this squarely since my more elliptical answer fell flat.
If you are well known in a medium like radio, agents come after you. Mr. Sedaris had a lot of very very funny work heard on a syndicated radio show called This American Life. I'm guessing he didn't write a single query letter about collecting those essays in a book, or writing a memoir. I'm guessing agents were after him from the moment they could start speaking again after laughing so hard they required oxygen.
Agents are on the lookout for fresh and new. My point about mentioning the radio show is that it is entirely possible to excel in something and draw the attention of agents without writing a query letter.
This works particularly well with deadpan humor. Miss Snark has been shouting orders and issuing instructions for years but it wasn't till she posted it on a blog that someone said "hey, you should write a book". (Miss Snark is not writing a book. Killer Yapp required a confidentiality agreement when he agreed to move to Snark Central).
If you "write like David Sedaris" think about forms that showcase your work. A place I spend some time every now and then is called The Moth Storyslam. I really love these events and I've found some very interesting people there.