This is clearly a problem that a lot of programmers have and to which Google has yet to provide a satisfactory, supported solution.
The setMenuBackground() hack which Abhay Kumar and Nik Reiman posted is a good start but it either crashes or does not work on Android 2.3.
Please see my answer (Louis Semprini) in this stackoverflow question for a better commented and more refined hack that works on 2.1, 2.2, and 2.3 (and that should do no harm on 3.X, though we can never guarantee this):
How to change the background color of the options menu?
Also, here are many other resources you may find helpful for this question:
Change background color of android menu
Android: customize application's menu (e.g background color)
http://www.macadamian.com/blog/post/android_-_theming_the_unthemable/
Android MenuItem Toggle Button
Is it possible to make the Android options menu background non-translucent?
http://www.codeproject.com/KB/android/AndroidMenusMyWay.aspx
Setting the menu background to be opaque