Lion’s new Sandbox feature, which is incorporated into the WebKit framework and prevents malicious or faulty code from interfering with other running processes, has a confirmed bug where text may appears as block-A (a square with a capitalize A in it) when viewing web pages in Safari or Chrome. This bug seems to only occur if you are using a third-party font management utility like Suitcase Fusion 3 or FontExplorer X, due to the way they activate fonts outside of the primary three font locations.

This ultimately will need to be fixed by the third-party font management developer, so if you’re experiencing this bug make sure your font management utility is up to date and deemed Lion compatible. The only workarounds are to use a web browser that doesn’t use the WebKit framework, like Firefox, or temporarily disable your font management utility.