i'd install Logician's conditionals hack and do something like this in your register_customfields template:
Code:
[[($profilefield[profilefieldid]!=xx)]]
<tr>
<td bgcolor="$bgcolor"><normalfont><b>$profilefield[title]:</b></normalfont><br>
<smallfont>$profilefield[description]</smallfont></td>
<td bgcolor="$bgcolor"><normalfont><input type="text" class="bginput" name="$profilefieldname" value="$bbuserinfo[$profilefieldname]" size="$profilefield[size]" maxlength="$profilefield[maxlength]"></normalfont></td>
</tr>
[[/($profilefield[profilefieldid]!=xx)]]
[[($profilefield[profilefieldid]==xx)]]
<tr>
<td bgcolor="$bgcolor"><normalfont><b>$profilefield[title]:</b></normalfont><br>
<smallfont>$profilefield[description]</smallfont></td>
<td bgcolor="$bgcolor"><normalfont><select class="bginput" name="$profilefieldname"><option value="xx">Blah</option><option value="xx">Blah2</option></select></normalfont></td>
</tr>
[[/($profilefield[profilefieldid]==xx)]]