As an alternative to Hickson's trick, you can use the Universal Math Stylesheet (UMSS) with tbook. It has the advantage that you don't have to be able to install CGI scripts on your web server, and it is more universal. The disadvantages are that off-line viewing is a little bit inconvenient, and loading of the pages is slower.
See http://www.w3.org/Math/XSL/ for comprehensive information about the UMSS. There you can also download the four UMSS files. However for tbook, you only need pmathml.xsl and pmathmlcss.xsl. Copy both files in the same directory where the XHTML file is. This is also necessary for each copy of your document on a web server.
Then call tbook's XHTML conversion program tbtohtml with the XSLT parameter umss-rendering set to on. That's it.
Unfortunately Mozilla and Netscape use not-very-fast internal XSLT processors. Therefore a test document on my computer took 1.5 seconds to be displayed without the UMSS, and 4.2 s with it.
Other allowed values for umss-rendering can be found at http://www.w3.org/Math/XSL/ in the section “Specifying preferences”. They are used as values for the pref:renderer attribute.