It is some sort of bug for sure - I have the same problem you have on my MacBook pro, but works perfectly on the iMac, same settings obviously. I am sure a fix is on its way.
Nice summarised, Raju.
I did some more research on problem B. I am using an Apple Airport Extreme router with my iMac (working dictation) on Ethernet and MBP (with problem B) on Wifi. Taking the suggestion that it might be a LAN problem, I switched the connections over so that the MBP is on Ethernet and the iMac is on Wifi. No difference. Dictation continues to work on the iMac regardless of whether if its connection is through Ethernet or Wifi. Dictation continues not to work on the MBP.
I tried deleting com.apple.assistant.plist from /user/Library/Preferences, turning Dictation off and then back on again, and then rebooting the system. Still the same problem. I noticed that there is an additional file called com.apple.assistant.token.plist. On the iMac where dictation works, quick previewing this file shows a list of keys for the different languages supported. On the MBP where dictation is not working, a list of supported languages is shown, but critically, the keys are all missing. I did a Google on com.apple.assistant.token.plist and found that a similarly-named file on the iPhone that controls how the iPhone connects with Siri servers and that this file was introduced as a way to thwart people hacking Siri. I am suspecting whether if Apple may have coded some bugs inadvertently into dictation in ML to thwart dictation server hacking. This might be why my MBP is unable to generate the correct key to contact the Apple servers. Blame it on the people trying to hack Siri to work on unofficial devices!
I have deleted the tokens file away to see whether if the MBP will regenerate a new working copy but this has so far proved unsuccessful. As the keys are however tied in to indivual system, copying the file from the working iMac over to the MBP doesn't help. I have spent the last few nights trying to troubleshoot this problem with no success. C'mon Apple, what happened to "It just works" ???