Hi knobbygb,
I was in Las Vegas and Phoenix in June this year and certainly in Phoenix, America West were offering Travel vouchers of $300 for people on oversld flights ( not my flights).
Also in Las Vegas especially on Sunday ( lots of people leave and the queues for security were long)-when I was arriving I notticed a lot of flights with "oversold " on the monitors so no doubt compensation would be offered for volunteers ( do not know the exact amount).
Regards
TBS