I bet I know who that is, too. :-) When I got that one, I also had several fulls and partials out, so I wrote back thanking the agent but explaining that I couldn't grant exclusivity on the grounds that several other agents had it. I asked what the agent would prefer I do: send the manuscript now and promise to notify her immediately of any offers, or wait until everyone else had finished reading. Predictably she asked for the first option.
Some writers would advocate just sending it and explaining in the letter that you can't grant the exclusive but promising notification etc. I thought it was better to give the agent the choice so she wouldn't feel deceived if she didn't open the email right away and thought she had an exclusive when in fact she didn't.
