I found that if you modify this registry key:
: "D:\PROGRAM FILES\WINAMP\WINAMP.EXE" %1
(note that the %1 is gone)
that the Play button on the Logitech keyboard will open Winamp and start playing from the "default" play list, right where it left off when you closed Winamp, instead of opening Winamp and displaying the %1 in the playlist window. Note that I just did this for Winamp 2.91 but on another computer I have 5.0 working... I'm fairly certain I did the same thing to fix that one. Since it's with an iTouch registry key, it's probably the same fix for 5.04. (My iTouch version is 2.22.289 on both computers, both Black Elite keyboards, one is wired, other wireless.)
Of course, you should be familiar with the registry, and make a backup of the registry before editing it in case you need to restore it.
Hope this helps.