Having the same issue but can't seem to fix it. After various bits of Googling, I found this:
How to fix UK Android Market download and Gmail log-in errors | andrew | Google Android Blog)
Which suggests doing this:
So, I did that and it hasn't worked. Now I can't login to any Google account on my Hero using either username (@gmail or @googlemail); it says it ;can't establish a reliable data connection to the server' yet Internet is fine on phone and is connected to Wi-Fi connection. I have tried to find the option in Gmail on the Net to change it back to Googlemail (though I never actually actively changed it in the first place), but can't find it in the place the Google help tells me it is!
Any help or ideas that aren't a factory reset greatly appreciated people!
Thanks!