Can confirm this, working fine with a dongle, problem seems to be Marvell AVASTAR 350N.
Unfortunately Win8.1RT won't allow the installation of drivers for dongle...
That's a nuiscance for RT but we should not be in this predicament with PRO or RT. We shouldn't need to buy a wifi dongle when we have just spent a significant amount of money on a new surface pro with wifi.
Microsoft should fix the problem and release a patch.