Font problems with Dvips
Some fonts have been reported to produce wrong characters with preview-latex. preview-latex calls Dvips by default with the option -Pwww in order to get scalable fonts for nice results. If you are using antialiasing, however, the results might be sufficiently nice with bitmapped fonts, anyway. You might try -Ppdf for another stab at scalable fonts, or other printer definitions. Use
M-x customize-option RET preview-fast-dvips-command RETand
M-x customize-option RET preview-dvips-command RETin order to customize this.
One particular problem is that several printer setup files (typically in a file called /usr/share/texmf/dvips/config/config.pdf if you are using the -Ppdf switch) contain the G option for ‘character shifting’. This option will result in ‘fi’ being rendered as ‘£’ (British Pounds sign) in several fonts, unless your version of Dvips has a long-standing bug in its implementation fixed (only very recent versions of Dvips have).