I am having the same issue. I want to download Lion from the App store, however, I get "An unknown error has occurred" each time I try to log in.
The odd part is that I can log in fine to ITunes and ICloud, just not the App store. I'm running osx 10.6.8 on a MacBook Air.
Would be super-grateful if anyone had any tips/hints/ideas whatsoever. I've tried rebooting, etc. Thanks.