I spent an hour trying to find reasonable instructions on how to
install ruby 1.9.1 as a standalone ruby instance. Everything seems to
be directed at side-by-side 1.8/1.9 installation.

I found this in some ad swamp, but it was sadly more appropriate than
anything else I could find on the web that wasn't apt-get ruby.(1.8.7)

ruby 1.9.1 is installed in opt/ruby but the system doesn't know about
it.
opt/ruby/bin/ruby -v works, but ruby -v tells me ruby isn't installed.

I guess I need to put ruby in the path or create a symlink or
something, but I am not sure what to do.

Thanks in advance.

Here is what I did:

1: Download ruby 1.9.1, ftp://ftp.ruby-lang.org/pub/ruby/1.9/ruby-1.9.1-p0.tar.gz
( I actually got p243 at ftp://ftp.ruby-lang.org/pub/ruby/1.9/ruby-1.9.1-p243.tar.gz)

2: To ensure that the installation of the necessary tools
sudo apt-get install build-essential
sudo apt-get install autoconf

3:
tar xzvf ruby-1.9.1-p0.tar.gz
cd ruby-1.9.1-p0(actually p243)
autoconf
sudo makedir /opt/ruby
./configure --prefix=/opt/ruby --enable-pthread
make
sudo make install

4 Installed?
/opt/ruby/bin/ruby -v