Thanks Brian but that does not work with the version of RequestCenter that I am on. Sorry that I did not mention that we are using RequestCenter without a servicepack. I did find that it is a problem that was fixed in servicepack 6.
We've found an undocumented behaviour change between 2008.1 and 9 that caused us major problems after our upgrade where when you try and set a Person field to blank, unless it has 'edit' writes at the moment, it'll throw an error and essentially lock the form up (that is, no actions can be taken - done/update/approve/reject). We've got a case open for it.
We skipped 2008.3 unfortunately so I'm not sure if it's the same case, but you may need to make it editable for the conditional rule to run... Interestingly, you can still set other field types as blank, it's only Person that it affects (and we have a case open for it).