[calorie]

One idea would be to add a little JavaScript to the vB pm_newpm template.

From:

<div id="pmrecips"><textarea id="pmrecips_txt" name="recipients" rows="<if condition="is_browser('mozilla')">1<else />2</if>" cols="50" tabindex="1">$pm[recipients]</textarea></div>

To:

<div id="pmrecips"><textarea id="pmrecips_txt" name="recipients" rows="<if condition="is_browser('mozilla')">1<else />2</if>" cols="50" tabindex="1" onchange="if (this.value.toLowerCase() == 'username') confirm('Are you sure you are sending this PM to the correct person?')">$pm[recipients]</textarea></div>

Where you replace username with your actual username in lowercase.