Hi Victor,
I added support for this yesterday, it is in the repository now so it will be in the next release. You can add an attribute like this to enable it:
<portal:AnalyticsAsyncTopScript id="analyticsTop" runat="server" AnonymizeIp="true" />
Note also that even without this setting (ie in the current release of mojoPortal) you could directly add the needed script in layout.master by adding this somehwere inside the form below the top analytics control:
<script type="text/javascript">
_gaq.push(['_gat._anonymizeIp']);
</script>
Best,
Joe