It appears that I was mistaken about how LMCE was trying to connect to the mythconverg database. It wasn't using root@localhost after all. It connects as that user for other database stuff. For myth it seems to be using the username/password found in the /etc/mythtv/mysql.txt file. In my case this file had a goofy random password. I was able able to change it to a known value and then modify the password for that user in the mythconverg database to match. That seemed to resolve the connection issues. I also deleted the ~mythv/.mythtv and the ~root/.mythtv folders. These were hanging around from my previous myth install, and they aren't needed with Pluto.
Inside of LMCE I still can't watch TV. When I select media->TV the mythfrontend launches correctly. I'm able to see the main menu. As soon as I hit 'Watch TV' the screen goes black for a few seconds and then turns grey. Since I'm not seeing the "unable to connect to database" message anymore, so I'm pretty sure the frontend is able to talk to the backend. I also know that the ivtv drivers are installed correctly for my PVR-150. I can run 'mplayer /dev/video0' and see video just fine.
Here's the error that I'm getting in the MythTV-Plugin log when I try to watch live TV:
/01/07 22:17:09.977 Parameter 159(PK_Screen): 128 <0xa7389ba0>
08 04/01/07 22:17:09.979 Parameter 5(Value To Assign): MythTV Player(LinuxMCE LiveTV) <0xaab90ba0>
08 04/01/07 22:17:09.979 Parameter 9(Text): <0xaab90ba0>
08 04/01/07 22:17:09.979 Parameter 29(PK_MediaType): 1 <0xaab90ba0>
08 04/01/07 22:17:09.979 Parameter 41(StreamID): 2 <0xaab90ba0>
08 04/01/07 22:17:09.979 Parameter 48(Value): 0 <0xaab90ba0>
08 04/01/07 22:17:09.979 Parameter 50(Name): mythfrontend.Mythfrontend <0xaab90ba0>
08 04/01/07 22:17:09.979 Parameter 103(List PK Device): 24,24,15,,1,1,0 <0xaab90ba0>
08 04/01/07 22:17:09.979 Parameter 120(Retransmit): 0 <0xaab90ba0>
08 04/01/07 22:17:10.005 Received Message from 19 (OnScreen Orbiter / Family Room) to 28 (Photo Screen Saver / Family Room), type 1 id 193 Command:Off, parameters: <0x79b2eba0>
08 04/01/07 22:17:10.005 Parameter 97(PK_Pipe): 0 <0x79b2eba0>
05 04/01/07 22:17:11.186 Socket::ReceiveData 0x85ec888 failed, bytes left 0 start: 1060000 1: 0 1b: 0 2: 0 2b: 0 m_Socket: 140 Incoming_Conn Socket 140 127.0.0.1EVENT 24 T=35 Event #24 <0x7b331ba0>
05 04/01/07 22:17:11.207 Socket::ReceiveString2 ReceiveData failed m_Socket: -1 Incoming_Conn Socket 140 127.0.0.1EVENT 24 T=35 Event #24 <0x7b331ba0>
05 04/01/07 22:17:11.207 TCPIP: Closing connection to 24 (Router Dev #0) 0x85ec888 m_Socket: -1 <0x7b331ba0>
05 04/01/07 22:17:11.207 Router::RemoveAndDeleteSocket 0x85ec888 24 <0x7b331ba0>
05 04/01/07 22:17:11.186 Socket::ReceiveData 0x85fddc8 failed, bytes left 0 start: 1060000 1: 0 1b: 0 2: 0 2b: 0 m_Socket: 141 Incoming_Conn Socket 141 127.0.0.1EVENT 24 T=35 Event #24 <0x7bb32ba0>
05 04/01/07 22:17:11.209 Socket::ReceiveString2 ReceiveData failed m_Socket: -1 Incoming_Conn Socket 141 127.0.0.1EVENT 24 T=35 Event #24 <0x7bb32ba0>
05 04/01/07 22:17:11.209 TCPIP: Closing connection to 24 (Router Dev #0) 0x85fddc8 m_Socket: -1 <0x7bb32ba0>
05 04/01/07 22:17:11.209 Router::RemoveAndDeleteSocket 0x85fddc8 24 <0x7bb32ba0>
05 04/01/07 22:17:25.384 Router::RemoveAndDeleteSocket 0x8619208 24 <0x7b331ba0>
Based on the socket errors, I can only assume that the pluto frontend is not getting any data from the backend so it eventually times out. That's probably why the screen turns grey.
Does anyone have a clue what's going on here?