I've done a more recent test at Litmus, with data URIs for inline <img> elements and css background images.

These desktop clients do show data URIs:

  • Apple Mail 5
  • Apple Mail 6
  • Lotus Notes 8
  • Outlook 2003
  • Thunderbird 3.0
  • Thunderbird latest

These mobile clients do show data URIs:

  • Android 2.3
  • Android 4.0
  • BlackBerry 5 OS
  • iPad
  • iPhone 3GS
  • iPhone 4S
  • iPhone 5

None of the webmail clients showed data URIs.

These desktop clients don't:

  • Lotus Notes 6.5
  • Lotus Notes 7
  • Lotus Notes 8.5
  • Outlook 2000
  • Outlook 2002/XP
  • Outlook 2007
  • Outlook 2010
  • Outlook 2011
  • Outlook 2013

These mobile clients don't:

  • Gmail (Android)
  • Outlook.com (Android)
  • Yahoo (Android)
  • BlackBerry 4 OS
  • Symbian
  • Windows Phone 7.5