As you probably heard by now, the Mac App Store is live. It is bundled together with OS X 10.6.6 update. US$4.99 is the new US$0.99 in the Mac App Store. I have not gotten any paid app from there yet as I bought all the software directly from the software company itself and hence the Mac App Store is pretty useless for me in terms of paid app. I only wish you can transfer license for those Mac software that you have bought to the Mac App Store so that it will handle the updating for you. But right now that is not possible. The only way is you have to buy it again which is stupid. Lets see what happen to the Mac App Store eventually.
From Panic Blog  (Makers of Coda & Transmit)
If you’re a current customer, there’s no way to convert a previous purchase into a Mac App Store purchase — that requires a re-buy. But remember, what you have now will continue to work just fine.
The Mac App Store may show software bought from us previously as “Installed”, even though they’re two different licenses. You will not get Mac App Store auto-updates unless you purchase from the Mac App Store. To re-enable the “Purchase” button in the Mac App Store, just drag the app to the trash and empty your trash. Your preferences/sites will not be affected.
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Official Site: Mac: App Store