I can't find an easy way to modify this text, from within LimeSurvey:

Also, we would really like to remove the "first name" and "last name" boxes: all we want to collect is the email address. Is this possible?

We'd really prefer to host the survey on LimeService, so modifying code is out - but can do it if there's no other solution.

Ah, I've solved it.

1. Copy the default template
2. On startpage.pstpl, insert this at the end of the HEAD tag:
<script>
$(function(){
$("[name=register_firstname]").parent().parent().hide()
$("[name=register_lastname]").parent().parent().hide()
});
</script>
3. On register.pstpl, replace {REGISTERMESSAGE1} and {REGISTERMESSAGE2} with whatever you want. (This wouldn't work if you needed multiple languages.)

Sweet!