The issue with Italic (and possibly Bold as well) is a consequence of RT-17179.
02-02-2012
As I've seen for 2.1.0b10, issue is actual only in part of italic. Crossed, Underlined and Bold i've seen in b10.
26-01-2012
If it works on Mac and Windows, but not on Linux, then chances are that
the problem isn't the HTMLEditor control. but rather lower down in the stack.
Re-assigning as WebView for further evaluation.