I offer both PayPal and GoogleCheckout as payment option on my pounds sterling payment page (GoogleCheckout only allows me to price in pounds sterling, unfortunately). As GoogleCheckout is effectively free to me at present I put the GoogleCheckout button on the left in the hope of getting more payments through Google. But 70.5% of purchasers clicked on the PayPal button.
I have since then become a bit disgruntled with GoogleCheckout for their slow processing times, chargeback fees, lack of multi-currency support and use of anonymised email addresses. So I swapped the button order in the hope of increasing the number of purchasers using PayPal. 69.3% of purchasers now click on the PayPal button.
From this I conclude that GoogleCheckout still has a long way to go to beat PayPal in brand recognition, positioning on the left may not be more prominent (although 1.2% may be statistical noise) and button order is less important than I thought. Or perhaps the PayPal icon is just more compelling. I wonder if GoogleCheckout have tested their icon against the PayPal icon?
 Google currently process £10 of payments free for each £1 I spend on Adwords.
 The user can opt to have their email anonymised at time of purchase. The vendor then recieves an email address like Missemail@example.com. Google forwards email from this address to the purchaser, until they choose not to receive further emails. In theory this protects the purchaser from vendor spam, but in reality it makes support more difficult. For example, the purchaser can’t retrieve their key from your online key retrieval system unless they remember to use the anonymised address (they never do).