I sometimes find it useful to connect my iPhone to my Macbook Pro by creating an ad-hoc wireless network from the MBP and connecting to that from the iPhone. However, what I find is that sometimes this works and sometimes it doesn't.

When it's not working the symptoms are usually that I see the ad-hoc SSID in the list of available networks on the iPhone, can connect to it from there (including entering my WEP key), and it shows up as the wifi network in use. However I don't get the wifi symbol in the taskbar (it remains as 3G) and attempting to use the connection (e.g. trying to connect to iTunes or Keynote using their respective Remote apps) fails saying that there is so wifi connection.

I've tried rebooting both the iPhone and the MBP, recreating networks with different SSIDs and tried different channels - all to no avail!

I'm especially puzzled that (a) sometimes it works just fine first time and (b) the Settings app seems to think its all connected fine.

Is there anything else I should be trying?