Update: the iPad 2 behaves exactly the same as the iPad 1 in this regard. No peer-to-peer, ad hoc, computer-to-computer, networking at 5Ghz (channel #'s greater than 11).
(This is not a surprise, since the ifixit teardown showed that the wireless card is unchanged from the iPad 1 to iPad 2)
I guess I'll do a bug report, although this seems like something Apple must be aware of...
I am experiencing this,too. We use the iPad to connect to our devices (which have dual band wireless built in) for streaming video.
When using any of the 5GHz channels, the iPad typically refuses to connect to the network, simply saying that it is unable to connect. Sometimes, though, it appears to connect without issue, but never transmits or receives any network traffic. Trying to renew the DHCP lease does nothing either, no traffic is seen.
The same thing is observed when trying to connect to a network created by my Mac (both Mac Mini and MacBook Pro). Most of the time the iPad refuses to connect, sometimes it appears to connect but cannot communicate with the Mac.
Another thing to add, I have also experienced some strange connection issues with connections in the 2.4GHz range. Sometimes the iPad will connect in 802.11n mode, sometimes in 802.11g. When it does connect in the 'n' mode, the connection quality seems to be poorer (more dropped or out-of-order packets) than in the 'g' mode.
None of these issues are observed when using a wireless access point.
Here we are in June 2014 with an iPad (5) Air and still lots of problems between a MacBook Pro 15" Retina late 2013 and the iPad Air with 5GHz ad hoc network. I setup a channel 44 40-bit WEP ad hoc network on the MacBook and sometimes the iPad Air sees it and sometimes not and when it does see it all I get is an error when I try to connect to it. Channel 11 and below works fine of course. Apple needs to address this. Ad hoc networks are essential in electronic music between devices like the iPad and the Mac for low latency communication between the two devices.