Need help ID'ing this caterpillar

[Vincep]

I have two of these little fellers munching on a Plum tree
in my front yard (at least, I THINK it's a Plum tree... ?)
They're about 2.5 - 3 inches long...

Anyone have any ideas?   They're not threatening the
tree, at this point, so I'm inclined to wait and see what
they'll turn into.

picture at:
http://pw1.netcom.com/~vpearman/caterp.jpg

(Please reply on NG or to "vpearman (at) netcom (dot) com")

Thanks!

-Vince in Eldersburg, Maryland.