Concentration of caffeine in physiologically relevant setting
Let's figure it out. Suppose an adult with an average blood volume of 5 L (5000 mL) drinks a cup of strong coffee with 100 mg of caffeine in it. Given caffeine's formula weight of 194.19 g/mol, and assuming 100% of the caffeine is in the blood stream, its concentration (from this awesome calculator page from Graphpad, the makers of Prism) would be approximately 103 &micro;M, or 10-40 times less than what's being used in the experiments you cite.

Of course, this is overcoming drug-mediated cell cycle arrest, so its possible that a lower concentration is needed to jump-start physiologically-arrested cells.