Generally OW carriers do not offer alliance members consistent baggage allowances. My experience as AA PLT (Sapphire) on IB is that if you check in at Iberia Business Class check-in, they are pretty loose on the baggage limits. At least as far as weight goes, not sure about pieces. I've had good luck in the past and recently (Mar. 2010) flew with another PLT on IB and we checked in 4 pieces in total: 20 kg, 24 kg, 28 kg, and 31 kg (!) and weren't asked to pay anything extra. Normal limit would have been 4 pcs. @ 23 kg max. each piece. So we took a whole lot more than that. We were prepared to throw a bunch of stuff away if they wanted to charge us, but in the end we didn't have to. Thanks Iberia! YMMV

EDITED TO ADD: if you are really worried and are sure you are exceeding the limits with items that are indispensable to you, they do offer a way online to buy additional weight/pieces ahead of time and save money over what they would charge at the airport check-in. We almost opted for this since we knew we were over but decided we could offload enough junk to get in under the limit if they were enforcing.