both Darv and Arora vote barrowsii; obvious barrowsii fruit bodies have been collected this year in the same location; although the single fruit body was somewhat ambiguous, I am (finally) convinced that barrowsii is the best call.

Compare:

http://mushroomobserver.org/15488

I doubt B. regineus, the pileus just doesn’t look nearly dark enough.

B. barowsii is a good idea, it appears pale enough, at least from the photos.

My instinct (whatever that is) tells me this is B. edulis var. grandedulis, as I have collected many which are unusually pale, like this one. Especially, as noted, if they are partially or not emerged from the duff or ground.
The frosted cap may be a defining characteristic of B. regineus, however, it is not at all uncommon with B. edulis var. grandedulis.

…and this is one of the difficulties of trying to name a single, immature fruit body! David Rust (the collector) tells me that this mushroom was buried when he found it, which lends credence to Ron’s theory of it being an edulis(pigments in edulis are photosensitive). David also tells me that there WAS some pine around, as well as hardwoods (we were apparently discussing two different mushrooms and their habitats).

Arora states that cap pigment development in edulis is photosensitive; altho the cap color underneath the frosting was not white but pale brown, it was also not as dark as one would expect a regineus to be. The orange-background apical reticulations fit that species tho, as does the white cap frosting. If someone would tell me with some measure of assuredness that regineus pigments are also phototsensitive, then I would be comfy with my original ID. At this point, tho, I am harboring doubts, dammit.

Mushrooms Demystified has color photos of both species on the same page. Sure looks like the “white bolete” to me.

the cap was not glabrous and white, but frosted white over a darker brown. the reticulation was typical of regineus: close, restricted to the apex, and colored apricot. The habitat was typical of regineus as well, fruiting under madrone and tanoak, manzanita and scattered incense cedar (altho barrowsii primarily prefers conifers, its only known hardwood associate is live oak).
It was also in a VERY wet environment; supposedly barrowsii likes it a bit more xeric. I can see where a mere photo ID could be mistaken for barrowsii, tho.

The color is very pale on your big meaty specimen. Looking through the other obs here, Boletus regineus seems to have highly variable cap color.