Noticing Lars' missive just now: Oaks are certainly *not* a red herring for the conservation status of Slender-billed Nuthatch. Robust local populations are very strongly correlated to places where you have open-grown, older oaks (with lateral limb development).
Habitat structure is also important. But if you want to see these birds in the Willamette Valley, your best bet by far is to look in big old oaks.
If you spend a lot of time in woodlands of other composition, well yeah, you can find a few. But the number you find per hour of effort is generally going to be much lower.
Taking the "Luckiamute Landing" tract of Luckiamute State Natural Area as an example: Not a single report there, all of this year, has listed more than 2 White-breasted Nuthatches on a single day, despite that this contains one of the largest tracts of older bottomland "gallery" forest along the Willamette River, with good access along a trail that brings visitors within listening range of most of the habitat. In contrast, I can think of oak woodland sites under conservation easements where you can find 10 to 20 pairs in the space of a morning.
Exceptions to general "rules" are often what get birders' attention. These things stick in your mind, and then get recalled as anecdotes when topics like this come up. From a "birding" standpoint, of course these unexpected encounters can be fun. From a population conservation standpoint, their significance is doubtful.
As I expressed to Wayne, I'm always a little worried what message people take away from these types of discussions. I hope no one comes away believing that preservation of older oak woodlands is unimportant, just because the birds sometimes can be found in secondary habitats. Slender-billed Nuthatches reach their highest breeding densities in open oak woodlands, and those areas of high density are significant for the population. See e.g. [ http://oregonwildlife.org/documents/nuthatch_web2.pdf | http://oregonwildlife.org/documents/nuthatch_web2.pdf ] for more discussion and references to the relevant research.