Yes - same issue...I just called Apple Care and they had me reset my automatic settings (in network) to a manual 'new' location. Once this was done it 'so far' has solved that problem.
The next issue is that Mountail Lion caused my Microsoft Database to crash. I run MS Office for Mac and have had no issues for 15 months...now it crashed less than 24 hours after installing ML.
I am rebuilding using the MS Db utlitly...this is not good.
If you have problems...such as Mountain Lion causing network disconnects. Mountain Lion not getting ethernet while still be connecting to the network then please post...perhaps Apple will address the issue and send out a bug fix!