The 10 year US bonds haven't had such low rates and for such a long time at least since 1962 (as far back as the Yahoo Finance data goes). Inflation is low. Your belief in the valuation should be linked to how you feel about rates and inflation. If rates stay this low for a very extended period than the valuation might be on the low side. At any rate, these are unchartered territories in terms of investment returns and valuations.

EDIT: even going back to 1912 the 10 year rates are unusually low for an extended period. The closest historic period is during the 1940's valuations back then (P/E ratios) were lower (peaking around 1946).