It is not implemented in the current beta. You can of course use the comments field for tagging, using the “&” character before tags, for instance. There are many websites that talk about how to do this in OS X. You will be able to convert these tags when DT does implement them, and they are searchable in Spotlight and Quicksilver. Personally, I just put tags right in the text notes that I write with the “tag:nameoftaghere” method. Works brilliantly. You can use smartlists, etc with either method or use the search functionality. So, there are lots of workarounds possible.