Yeah - this is possible.
The template to change is skin1/modules/product_options/customer_options.tpl
There is html in there for the drop down box...this can be changed to a radio group html if you prefer.
Code:
{* $Id: customer_options.tpl,v 1.9 2003/11/14 11:06:33 svowl Exp $ *}
{section name=product_option loop=$product_options}
<tr>
<td valign=middle height=25>
{if $usertype eq "A"}
{ $product_options[product_option].optclass }:
{else}
{ $product_options[product_option].opttext }
{/if}
</td>
<td valign=middle>
{if $product_options[product_option].options ne ""}
{assign var="options" value=$product_options[product_option].options}
{*
<select name="{if $cname ne ""}{$cname}[{$product_options[product_option].optclass}]{else}product_options[{$product_options[product_option].optclass}]{/if}"{if $disable} disabled{/if}>
{section name=option loop=$options}
<option value={$smarty.section.option.index}{if $options[option].selected ne ""} selected{/if}>{$options[option].option}{if $options[option].surcharge ne 0} ({$options[option].surcharge|value_sign:"1"}{if $options[option].type eq "absolute"}{$config.General.currency_symbol}{$options[option].surcharge|abs_value}{else}{$options[option].surcharge|abs_value}%{/if}){/if}</option>
{/section}
</select>
*}
{section name=option loop=$options}
<label><input type="radio" name="{if $cname ne ""}{$cname}[{$product_options[product_option].optclass}]{else}product_options[{$product_options[product_option].optclass}]{/if}" value="{$smarty.section.option.index}"{if $options[option].selected ne ""} checked{/if}">
{$options[option].option}{if $options[option].surcharge ne 0} ({$options[option].surcharge|value_sign:"1"}{if $options[option].type eq "absolute"}{$config.General.currency_symbol}{$options[option].surcharge|abs_value}{else}{$options[option].surcharge|abs_value}%{/if}){/if}
</label>
{/section}
{else}
<input type=text name="product_options[{$product_options[product_option].optclass}]" size=32>
{/if}
</td>
</tr>
{/section}
{if $err eq "options" and $options_ex ne ""}
<TR>
<TD colspan=2><FONT class=CustomerMessage>Note: this product cannot be added to the cart with the following options combinations:</FONT></TD>
</TR>
<TR valign=top>
<TD colspan=2>
{section name=oi loop=$options_ex}
* {$options_ex[oi].exception}
{/section}
</TD>
</TR>
{/if}