jmcdermott comment as otherwise you would see the MAC Address of the iPad Device(s) in question.
Do you have any authentication ie certificate required or anything like that or is it just a WPA2 encryption with the relevant key etc or what exactly ?
This is a strange issue as there is nothing complicated about the connection process. Nothing changes on the Meru Wireless (nothing needs to as it works) - I only log in once a week to check logs etc so I know the config hasn't changed.
If you've got a MacBook Pro to hand, you could sniff the iPad's connection attempts and compare to some that work.
Edit: ... sniff... with wireshark. (iPhone app crashed and had to redraft this post, and then I went and forgot to mention the all important name of the tool!)
If the problem isn't faulty iPads/ios config , a reboot of the controller and the ap's should fix it.
A reboot of the controller and ap's has resolved this issue. The affected iPads are connecting fine this morning.
Thanks for all the suggestions guys - good to know i'd tried almost everything before resorting to a reboot.