The methyl radical prefers a planar structure. The energy barrier to pyramidaliation is quite low though. The methyl radical is really the only totally planar alkyl radical. Other structures pyramidalize to avoid eclipsing in interactions. so to answer your question, the methyl radical is sp2 but that cannot be directly extended to other simple alkyl radicals. This is more of a academic excerise though, and when discussing reactivity etc, planar seems to work out okay. Hope this helps.
JACS, 1981, 103, 5046