According to Rixstep.com Apple plans to set up an App Store starting with the introduction of Mac OS X 10.7.

Developers planning on marketing software for 10.7 will submit their products to the App Store as iPhone and now iPad developers have already done. 10.7 will have kernel support for (‘insistence on’) binaries signed with Apple’s root certificate.

No software will be able to run on Mac OS X 10.7 without being approved and signed by Apple, Inc.

According to MacStories.net allegedly Steve Jobs has answered a direct request send to him by Fernando Valente of Chiaro Software. Valente asked him:

There’s a rumor saying there will be a Mac App Store and no software without authorization from Apple will run on Mac OS X. Is that true)

According to the mail Steve Jobs (or someone managing his email account) simply answered:

Nope.

Anyway: you cannot be sure what his answer specifically means. Probably we’re gonna need jailbreaks for Mac OS X in the near future anyhow…