First, thank you to everyone who replied, either on or offline.  As with everyone who replied, I'm now certain that it's a Willow Flycatcher.  For the record too, Derek Richardson had a Willow Flycatcher on the Trail in May/June 2005 (2 observations each).

I don't know if anyone else managed to see it since Saturday afternoon, but it was still there around 8:30am this morning.

While I'm not ready to say this conclusively. I have a suspicion that there may be two of them (possible breeders?).  When I went back on Saturday to record it (audio), the one I saw was perched high in a tree singing.  It flew to another tree and continued singing for about one minute.  Then it flew off as another bird of the same size flew toward it and quickly disappeared out of sight.  A few minutes later I heard the calling again from behind some trees.

This morning when I first heard it again, and while looking for it, I saw a bird that looked like the willow with something in its mouth.  I didn't have my binoculars out at that point (my mistake), but saw another bird nearby and before I knew it both were out of sight.  Less than a minute later, one was up on high singing in plain view.  Listened to (and again recorded) this individual.

I'll try to get more conclusive evidence.

Also, a few people wondered the closest place to park.  If you look at that map I posted (http://tinyurl.com/willowalderPBT), I added 3 RED Markers for potential places to Park.  You can park by the Tennis courts on Cherry Hill or in the small parking lot alongside the trail near Acredale Park (Metzerott Rd. just south of 193).  The golf course/driving range also has a parking lot, which is a tad closer than the one off Metzerott, but it's not certain whether it's "public" or not.

Good birding.
Ryan Farrell,
College Park, MD