The N900 works without SIM - I used mine for two weeks before I inserted a SIM at all. It works just like the N800 without SIM. So if SIM was a problem it should work by removing the SIM, which you did. Reflashing should also help removing any issues (although it doesn't reflash _everything_ - in particular there are bits and pieces in the GSM card chip which, if updated, won't be restored by a reflash. But wi-fi shouldn't be affected by that).

To me it sounds like a warranty issue. To be certain, when you get your replacement N900, try it first before putting in a SIM card. It should work just like your N800. If, after that, installing a SIM card destroys wi-fi then we know something that we didn't know before. But it sounds unlikely.