I got a response from one of the agents I queried this week.  The subject line was the word "Representation."  (Actually, that was the name in the "From" line, but I didn't notice that distinction at first glance.)

Okay, so I knew I'd only sent a query and it was therefore impossible that anyone would be offering representation based on that.  But it still made my heart beat a little faster. 

The body of the email said this: 

Dear Ms. Scott:

Thank you for writing to inquire about my interest in representing your

project, /Scratching at the 8-Ball/. Unfortunately, I must decline because I cannot take on the work of additional clients at this point.

My best wishes as you seek publication.  Sincerely,  [Agent]