The same for me.
I cannot access itunes store since 2 days, more or less... before that date everything worked fine.
This happen only with my MacPro, OSX 10.6.8, iTunes 11.0.4... if i try with my old macbook white, OSX 10.5.8, itunes 10.6.3, everything works fine (same network and ISP).
I tried also a different network (3G), but with same results.
just solved mine - everything works fine now.
Found this page; http://support.apple.com/kb/TS4123?viewlocale=en_US
and one of the listed programs was installed on my computer,
so when I uninstalled, problem solved!
Hope it works for you too!
Finally I can use my vouchers after nearly a year of frustration!
I have the same problem and the same setup - MacPro 10.6.8 and iTunes 11.0.4.
I tried two things
1. updated to iTunes 11.0.5 which didn't fix the problem.
2. updated Safari to 5.1.9 which fixed it....
So I'm not sure if you need iTunes to be at 11.0.5 or 11.0.4. but it appears Safari was my problem....