This morning, 5/1/12, I heard a probable Bell's Vireo at Oak Park.
Along Tallant Road, just east from where it passes by the tennis courts, there is a staircase. I heard the bird between the stairs and the Samarkand community.
I never saw the bird (I needed to get to work) and although the song sounded a bit slow for Bell's, it sounded much to scratchy for Warbling. I only heard it call twice.
Santa Barbara, CA