On or around Sat, 09 Jun 2007 14:15:27 +1200, EMB enlightened us thusly:
they do have rather different wheelnuts.
but in principle, yes, there's nothing wrong with alloy spacers if the spec and manufacture is right, and it should be if it's being sold in a shop.
I'd be a bit iffy about the amount of material in a thin (e.g. 30mm) spacer made from alloy - by the time you counterbore it enough to allow fitment of a credible sized wheelnut, there's not a huge lot left. But with the right spec alloy, I dare say it's do-able.