I thought that in “classical” vampirism, vampires did cast reflections, but avoided mirrors because it cast their true reflection (a wizened dead beasty) rather than the charmed/enchanted handsome face. (Admittedly, that was before “Moisturize! Moisturize! Moisturize!” replaced “drink from me and live forever” as the secret of eternal youth.)
Spike actuallly does seem to have a certain tolerance for sunlight, at least indirect sunlight. He was actually sitting in the shade behind the Magic Box in broad daylight at the end of “After Life”. Buffy comments on him being out in the daytime and he says, “Well, the sun’s low enough, and we’re in the shade”. Also, in “The Gift”, the sun was just barely starting to come up, it wasn’t full daylight in that final scene. so Spike still would have had some time to get under cover. And in “Lover’s Walk”, when he was passed out in Angel’s garden, the sun came up and nothing happened until a bright ray of direct sunlight touched his hand and it burst ito flame.
'Course, if you want to ask Joss how come Spike casts a reflection and Angel doesn’t, he’ll probably come up with some lame-ass explanation like “It’s the curse, Spike’s soul is more firmly attatched to the rest of his being than Angel’s is.”
I missed that reflection thing in “Him”, I hope they rerun the ep during the post-sweeps hiatus so I can keep an eyeball out for it.