ColdFusion 9.0.1 introduces a small bug into cfdump where changes to function arguments will not show up in cfdump if the dump has already been run once. I don't know the background but my guess is that in order to speed things up CF is using the cache to store the dump of function code.
The fix to this is to clear from cache anything that starts with _cf_dump with this code:
<cfscript> for ( key in cacheGetAllIds() ) { if ( left( key, 8) == "_CF_DUMP") { cacheRemove( key ); } } </cfscript>
If you are doing a lot of looking at cfdump after changing methods (something I almost never do) then you can place that code in Application.cfc onRequestEnd.