If I build a website that uses CSS positioning to place content in the source code in a different place to where is appears within the browser (although still visible), what are Google's views on this?
are you asking how google feels about this or how the google algo reacts to this?
the google algo typically places more importance on what appears towards the top of your page code. so what you are doing will likely have positive results for you.
google is trying to improve their algo to better deal with this issue. in the meantime google knows that some webmasters abuse this. they are never happy when people abuse their algo. i wouldnt feel comfortable if google manually reviewed my site and saw this.