What you heard is outdated and applies (only partially) to Ruby 1.8 or before. The latest stable version of Ruby (1.9), supports no less than 95 different character encodings (counted on my system just now). This includes pretty much all known Unicode Transformation Formats, including UTF-8.
The previous stable version of Ruby (1.8) has partial support for UTF-8.
If you use Rails, it takes care of default UTF-8 encoding for you. If all you need is UTF-8 encoding awareness, Rails will work for you no matter if you run Ruby 1.9 or Ruby 1.8. If you have very specific character encoding requirements, you should aim for Ruby 1.9.
If you’re really interested, here is a series of articles describing the encoding issues in Ruby 1.8 and how they were worked around, and eventually solved in Ruby 1.9. Rails still includes workarounds for many common flaws in Ruby 1.8.