I know you said you're partial to whites, but I couldn't think of any outstanding whites from when we visited Walla Walla in 2004. So, here's my two cents.
K Vinters is making outstanding syrahs, which according to a story on NPR this weekend, are about to be the next "big thing" in wine. The vintner used to manage rock bands or something like that, so he is rather irreverent for a wine maker (the website used to have pictures of him in a Ramones shirt). But his wines are scoring very well and getting talked about. They are on the pricier side, but we splurged and brought some home and we're happy to have it to enjoy.
He also makes wine under the label The Magnificent Wine Company - perhaps you've seen this wine in your grocery store? it has a plain white label with black print and a black outline of a house that says "House Wine". For $10, this is a great wine for every day enjoyment.
The website says 'by appt only' for tasting, but don't be intimated. Call and ask if you can drop in.