I don't know if the "i" products support this but my laptop wifi card has settings for roaming aggressiveness. These can be changed to cause more or less frequent checks for signal strength and therefore WAP associations.
Update: after a little reading I found others with the same issue. One fix suggested by Apple is to force the device to renew it's DHCP lease.
[Link: makeuseof.com]
Last edited by jcbremotes on July 27, 2011 16:00.