I've seen several reports about the beta version of the Wacom driver for the Surface Pro being available. Our own Paul Thurrott even talks about it in his latest weekly column, Short Takes: Short Takes: May 10, 2013.
As stated, the Wacom driver is in beta and brings pressure sensitivity functions to applications that support that feature – applications like Adobe Photoshop. But, finding the driver takes a bit of extra effort because Wacom is lumping the Surface Pro into the same old Tablet PC category.
So to find the driver for download, jump out to the Wacom driver site:
…and, filter for "Tablet PC" and then "Windows 8"
The driver you want is the TABLET PC – Enhanced Tablet Driver 7.1.1-12
If that's even too much for you, you can download the most current release (as of May 10, 2013) at this link:
NOTE: I'm only providing this direct download link for convenience. I'm sure the download's filename will change as it progresses through beta, so make sure you visit the Wacom driver site to ensure you have the latest download.
UPDATE: Thanks to Windows IT Pro member, prettyconfusd, for some additional information! If you drop down and take a look at the comments below, you'll see that Wacom has a special page just for the driver. The driver page is: http://www.wacom.com/feeldriver