iChat uses CSS markup to denote style and font changes in messages, as opposed to tags. Adium currently does not support or does not properly process CSS styles.
XML Console output shows this when receiving text from iChat:
<message xmlns='jabber:client' to='sastira@gmail.com/AdiumE2496988' type='chat' id='iChat_39EBC8CC' from='jxxxxx@gmail.com/Apophis58DF4D67'>
<body>How about inline styles in addition to something in code?</body>
<html xmlns='http://jabber.org/protocol/xhtml-im'><body xmlns='http://www.w3.org/1999/xhtml' style='background-color:#C0E668;color:#000000'><span style='font-size:12px; font-family:Helvetica; '>How about <span style='font-weight:bold'>inline</span> styles <span style='font-style:italic'>in addition to</span> </span><span style='font-size:11px; font-family:Monaco; '>something <span style='color:#A90E93'>in</span> code?</span></body></html>
And this when sending text back out:
<message type='chat' id='purple12254289' to='jxxxxxx@gmail.com/Apophis58DF4D67'>
<active xmlns='http://jabber.org/protocol/chatstates'/>
<body>How about some italics and a bit of bold?</body>
<html xmlns='http://jabber.org/protocol/xhtml-im'>
<body xmlns='http://www.w3.org/1999/xhtml'>How about <em>some italics</em> and a <span style='font-weight: bold;'>bit of bold</span>?</body>
</html>
</message>
