I understand how to use sprites, however, isn't a "src" attribute required for IMG tags? I could always use a SPAN or other tag and set the background/width/etc but it won't be semantically correct.
Basically, I'd like to omit the SRC for an IMG tag and use just sprites, but am concerned about the HTML not being technically valid because of it.
About semantical correctness:
When an image has semantical meaning, so it is considered to be content, use an IMG tag, without sprites, and a correctly set up ALT.
When an image is just decoration, like the background of a box, background of a button, background of a menu option, etc., it has no semantical meaning, so you can just use it as a background of a SPAN, DIV, etc. from CSS. You can use sprites in this case.