ARP-spoofing can be detected although I'm not sure how many networks actively do this. Being able to decrypt all internal packets passively is a big deal.
It's a big deal - but most places that concerned about security, the ones that worry heavily about arp-spoofing and detection probably don't allow wireless in the first place..... at least that's been my experience.
ARP spoofing actually in practice not easy to do. Enterprise class switches can snoop DHCP traffic and prevent forged ARP packets.