In Rules, how can I use a set the value of a taxonomy term reference field to multiple values?
As it turns out, this is actually fairly easy to do in Rules.  By updating to the latest version of the Entity API (rc3-- I was stuck on rc1 due to a different bug that I resolved earlier today) with Rules 2.1, I was able to do this in the following way:


Choose the action "Set a data value".
Choose the taxonomy term reference field to set.
Control-click to select the taxonomy terms to add.


That's all there is to it, but there is one potential gotcha: these values didn't always save correctly for me.  This may be because I opened multiple tabs in Firefox and tried to save many different rules at once; for whatever reason, sometimes only one of the two values got saved.  I fixed this by simply checking all the values again manually and re-saving them until the values stuck.