EA are planning to bring Mac support to Origin, and are accepting testers to help trial their client. As an incentive, they're offering a free copy of Popcap's Bookworm to anyone who takes part. Applicants had better really like Bookworm, though, because the Origin store isn't due to go live on Mac until the client's official release.
Origin for Mac aims to bring all the features of the PC client - including cloud storage and auto-patching - to Apple's OS. Right now, as well as the Store being MIA, the Mac version is also missing Origin's nifty Twitch.tv streaming feature. Again, it's probably for the best. I'd imagine there are only so many Bookworm livestreams that I could stand to watch in a day.
EA assure that when it does properly launch, the Origin Mac catalogue will feature a selection of EA and third party games. According to their announcement, "We're very excited about the upcoming launch of Origin for Mac and the opportunity to reach the millions of Mac-based gamers out there. We're continuing to build Origin into a gaming service that truly lets you purchase and play anywhere, anytime."
The alpha trial is only available to a few thousand testers, but until that cap is hit, you can download the Mac client from here .