Jamie has included some great suggestions. All of which I recommend over this one, but most people don't realize that there is limited string manipiulation possible using JScript functions (i.e. substring and indexOf) within DWT TBBs. So this is possible:
@@"chris@urbancherry@@"chris@blah.net".substring(0,("chris@urbancherry"chris@blah.net").indexOf("@"))@@<br/>
@@"chris@urbancherry@@"chris@blah.net".substring(("chris@urbancherry"chris@blah.net").indexOf("@"))@@
Of course you can use a field value instead of the whole string, but this shows the point. I think this is pretty ugly, and would really suggest creating a Function Source, but in a pinch, this would work for you.