I just noticed an interesting thing with my iPad and iPhone that are often connected to my MacBook Pro 13". It appears that only one USB port can be in full charging mode and that the order that you connect the devices is important.
If I connect the iPhone followed by the iPad, the iPad will not charge (or rather it goes into the trickle charge mode with the standard USB power draw). However, if I connect the iPad first followed by the iPhone, both devices will charge just fine.
There seems to be some kind of internal power switching decisions being made by the MacBook that will allow the 10W draw on one port, but it seems that the iPhone requests more power as well. Interestingly, external USB powered hard disks do not provoke the same behaviour. I can connect a hard disk, followed by the iPad and it will charge just fine.