I am wondering weather or not to put a a "Call Now" button with our number in the top left corner of our page. This would move keyword-rich text to the left, giving it less weight. How much weight does Google give to the top right corner compared to the top left corner?
Google used to like more the links on top of the HTML structure (usually top-left corner), but nowadays it can read more of the page (usually all of it). As mentioned in one comment above by @Fiasco Labs it was worthwhile to position the navigation menu (for example) up in the HTML, and move it around with CSS if necessary. For some things I still think a correct positioning in the page it's important, but it's not related to this question.
I would think there is no importance difference between top-right and top-left corners. However it could make a (slight) difference if you do not use meta description, as Google would make up its own snippet for the SERP, therefore content higher in the HTML has higher chances to be in there (although Google makes up the snippet based on the searcher's query, so it could get the snippet from anywhere in your page or ODP).
Since it's a call to action button you're moving and with it some text and trying to determine if it's better to be in the top left or top right why don't you run either a split test or multivariate test in Website Optimizer and see for your self which placement performs better.
You're question is most likely about SERP's but if it's a call to actions placement on a page you should be more concerned on where better to place it for higher conversions than will it affect your SERP's