Yesterday I set up a Google Checkout account.
I then attempted to buy something at buy.com.
When I got to the buy.com checkout page, I then pressed the Google Checkout button.

From Mozilla on Win XP:
The window clears and displays something like "processing...".
After a few moments an alert comes up saying:
"Document contains no data."
From IE it gave some other error message.

Today I tried and after "processing..." the page displayed text complaining about Javascript possibly not being enabled, even though it was. It then gave the same alert.

I took a look at the html on that page.
The following was very suspicious:

<input type="hidden" name="cart" value=(this field had 39999 bytes) id="Hidden1">

When I was able to call buy.com they couldn't help me.
I sent an e-mail to checkout-supportgoogle.com, but have had no reply.
In the mean time I am not able to complete my transaction.

Perhaps someone reading this has a contact in Google that can do something in a timely fashion.

I had the same issues. You need to delete your cookies, and then it will work.

Google checkout support sent me an e-mail suggesting that and also clearing my cache.
I tried both along with rebooting, but it didn't help.
I may try a different computer tomorrow.

I used a different computer today. The checkout worked.

The problem from the original computer is not fixed.
I am awaiting a reply from checkout support.