I remember downloading a Matrix theme off wincustomize.com a while back, it was fairly well done, but I found it annoying cause of the animations. As far as AIM, it will skin that too, but i use GAIM on my laptop and Trillian on my desktop, I think they might have some skins that fit your description. As far as a wallpaper for the Matrix, I made one a while back for a photoshop class I was taking.

The big version.. and it is big (1600 x 1200) can be found here

http://69.44.59.200/~pmcgowan/Backgrounds/MatrixVortex.jpg

and a smaller preview here (640 x 480)

http://69.44.59.200/~pmcgowan/Backgrounds/MatrixVortexSmall.jpg

Just for you folks I removed the "vortex" stuff and it is here

http://69.44.59.200/~pmcgowan/Backgrounds/Matrix.jpg

These filenames are case sensitive
I hope you guys like and I hope I was of some help.

-mental