I can't see what I type as I write this question in Safari – all I see is lots of capital A's in boxes. I'm new here, so I'm not allowed to post a screen grab. No problems in other browsers, or in other apps. Seems to affect all the relatively recent Microsoft "C" fonts, but not Arial. I am using Font Explorer X Pro, which has exhibited a bit of post-Lion flakiness, but given it is apparently limited to Safari this is quite possibly irrelevant. Anyone else had this problem? Any ideas?
This is a known bug that has been reported widely.
First, if you are using a third-party font management program (commonly used by graphic designers with large libraries of fonts) such as FontExplorer or Suitcase Fusion, disable that utility. Contact the publisher of that font utility for information about an update or workaround to deal with Lion and/or Safari 5.1 incompatibility problems.
Second, try clearing your font caches. One way to do this is to boot in Safe Mode. Start up and as soon as you hear the startup chime, hold down the Shift key until the operating system starts to load (after the grey Apple logo, when the rotating 12-point clock symbol appears). It will take longer than usual to start up because the Mac will be doing some diagnostic and maintenance work, including deleting your old font caches so the system can build a new one. Once startup is complete, and you are back to the Desktop and Finder, you should restart, because some functions won't work while the Mac is running in Safe Mode.