Microsoft released a new version of the Windows Phone SDK today, or rather, released a quick patch that included a new emulator to test applications against to ensure they would run under 256mb of RAM. I haven't had much time today to look into things, but here's my initial analysis after dumping and comparing it to Mango's emulator:

• The new emulator is build 7.10.8731, which is a few builds back from the 8757 that's floating around. (8757 was staged and given to a select few devices worldwide. DFT happens to have it laying around)
• There are 2549 total files in the Tango dump that are different compared to corresponding files in the Mango dump. This number is particularly high for a variety of reasons, including regional/language file changes.
• The following files were added to the Tango dump (e.g. were not in the Mango dump)
  • 1fbb0a6b-4784-4626-967c-f854eb014585.dsm
  • 1FBB0A6B-4784-4626-967C-F854EB014585.rgu
  • 7c926679-58f4-4c18-b65f-0f9f21ae8290.dsm
    887d2d4b472246aea25c8fd2be91bb4e.xap (Help and howto)
  • 887d2d4b472246aea25c8fd2be91bb4eLicense.xml (License for above)
  • CopyPaste.wav
  • eccgen.dll (Something to do with encryption)
  • fmrxtestconfig.xml (Radio stuff)
  • FmStubPdd.dll
  • Internet Connection Sharing stuff
  • IcsClient.dll
  • icscsp.dll
  • IcsService.dll
  • intshare.dll
  • ipnat.dll
  • mpapHelpHowTo02.provxml (These allow the ConnectivityUX and HelpHowTo apps to be installed on coldinit (hard reset))
  • mxipupdateConnectivityUX100.provxml
  • mxipupdateHelpHowTo02.provxml
  • omadsprc.exe
  • SelfRegistration.dll (Microsoft Self-registration Message SMS Protocol, no idea)
  • ShortMsgProvider.dll
  • SIMContactsManager.dll
  • SimContactsSync.dll
  • softap.exe (IC settings)
  • softapuxdll.dll
  • SAPUXPacMan.xml
  • (Edit: These are present in Mango, but not in the emulator dump. Thanks @jessenic)
• The following files were removed from the Tango dump:
  • 6a540b21-b5c0-4d24-8903-b5b5eb97df58.dsm
  • ContactsSimImport.dll
  • HelpHowTo.xap
  • HelpHowToLicense.xml
  • Microsoft.Phone.xamlbin
  • Mosaic.wma (unpopular ringtone?) 
  • (old version) mpapHelpHowTo01.provxml
  • retaillog.dll

I'll start digging into the significant changes, but here's a little part one for people who are curious. Questions?