I don't know about MacBook Air. However, for the MBP 17" (2010 model), it charges the iPodTouch only when the computer is NOT in sleep mode. At first, I thought the computer charges the IPT even during sleep (as indicated on the IPT). However, sometimes the IPT lost battery power very quickly. After talking with the Genius, we found that when sleep, the current from the USB port is not strong enough to charge the IPT properly. So, she suggested me to charge the IPT while keeping the computer on. Since then, the charging works fine.