Hi Guys,
I came across this website which has the steps on how to use the shake feature in w580i without pressing the walkman button.
Here it goes.
1. Press Walkman (W) button.
2. Press Play.
3. Press down arrow (V).
4. Press right arrow 2 times (> >).
5. Press Walkman (W) button again.
6. Shake the phone left or right.
Shortcut - ( W Play V > > W ) - Shake shake.
Song changes. Cool isnt'it?
Source - http://walkmans.wordpress.com/
Video tutorial - YouTube - Shake control trick on sony ericsson w580
Hope all like it.