[xgarb]

My simple way of doing this (appears to work so far)

In /main/register_account.tpl

change

<TR>
<TD align="right">{$lng.lbl_username}</TD>
<TD></TD>
<TD nowrap>
<INPUT type="text" name="uname" size="32" maxlength="32" value="{$userinfo.login}">
</TD>
</TR>

to

<TR>
<TD align="right">{$lng.lbl_username}</TD>
<TD></TD>
<TD nowrap>
<INPUT type="text" name="uname" size="32" maxlength="32" value="{$userinfo.login}" onchange="javascript: checkEmailAddress(this);">
{if $fillerror ne "" and $userinfo.email eq "" && $default_fields.email.required eq 'Y'}<FONT class="Star">&lt;&lt;</FONT>{/if}
</TD>
</TR>

You should also change the string for $lng.lbl_username to something like 'Email Address'. Easiest to do in webmaster mode I find.

This forces the user (using javascript) to put in an email address as their username.

HTH

xgarb