What is the origin of the "r" in resistance measurements?
You are correct about it standing for a decimal point. So 2R2 means 2.2&Omega;, 0R5 means 0.5&Omega;, and so on. Also as pjc50 notes, it's common to see e.g. 47R used for 47&Omega; since using an Omega symbol is more difficult and time consuming (e.g it's a special symbol you can't just shift + something for on most keyboards)
Similarly, it is common to see 2K2 for 2.2K&Omega;, or 3M3 for 3.3M&Omega;

As far as I know, these conventions started back in the days when paper schematics used to get copied a lot (e.g. photocopied, faxed, etc) and the reproduction quality was not very good. This meant that small decimal points could easily get lost or even "appear" if a speck of dust was on the sheet at the time of copying.
So to make things clearer and these type of errors less likely, this type of notation was used.

Here are a couple of example clips from old schematics:

One where they should have used the notation discussed (60's guitar amp schematic - is that .02uF or 2F? ;-) ):



Another bad one:



Two clips from a better schematic from the mid 70's with the same convention used for capacitance also, plus nanofarads are used (in earlier schematics 22nF would be presented as 0.022uF):





Schematics found at Dr.Tube and Vox Amps. It's interesting to browse through the schematics and note the differences in notation over time.