This isn't so much a solution as a commiseration, but I've experienced this intermittently since upgrade to Go 12.0.3 a few days ago. Quitting Go by either opening other Aps until it actually closes, or by rebooting the phone brings relief. For a while.
In the absence of something more complete, I hope this at least helps.
May 23, 2012 @ 10:37 AM PDT Update.
1. I have verified that the iPhone is using the same network as the iPad 2, but without success connecting to the host server. I can see the server name, just not connect to any of the databases. When I use either fmnet:10.0.1.10 or http://10.0.1.10, I can't see the the FMServer_Sample.fmp12 using Safari from the iPhone or iPad. So there is some connectivity issue that I don't understand.
2. I am in fact using "FileMaker Go 12 for iPhone" on my iPhone 4. The version is 22.214.171.124120 as confirmed in iTunes after synching. I have re-installed the app several times on the iPhone and reset it, but still no success. I have remove FM Go version 11 from the iPhone 4. No other versions of FM Go are installed on either the iPad or iPhone.
3. Using the Network Utility on my Mac, I performed a port scan with these results:
Port Scan has started…
Port Scanning host: 10.0.1.10
Open TCP Port: 5003 fmpro-internal
Port Scan has completed…
So, I know that the files are being served. Also, my iPad 2 can see and connect to them.
4. I have used Activity Monitor to force quit all "fmserver" processes and restarted my Mac. Still no success.
5. I have uninstalled FMServer 12 and reinstalled it. Still no success.
6. I have reset the PRAM on the Mac and restarted. Still no success.
Any other suggestions would be greatly appreciated.
PROBLEM RESOLVED. Intego's VirusBarrier X6 had firewall settings that were preventing the connection. Disabling the firewall allowed me to connect from the iPhone 4. (Curious how the iPad2 was able to avoid these firewall barriers and not the iPhone 4.) Now, I need to configure out the best way to configure my firewall, whether through VirusBarrier or otherwise. Thanks to all who provided input.