I think most current production rifles are perfectly capable of MoA accuracy, including Art's citation of "all day long" indicating repeatability (not a a literal marathon shoot).
However, most shooters are not capable of the same.
They either don't care, have poor technique, or don't practice enough. (I'm drifting farther and farther into the latter bracket.
The majority of statements I see along the lines of "1 MoA all day long" are referring to the rifle, and sometimes the ammunition. Almost never, do you see some one say "I
can shoot 1 MoA all day long."
It's a statement about the rifle and/or ammunition's consistency and repeatability; not the shooter's abilities.