Today I finally closed bug 203706, 4 years after it was initially opened. That means that you can now go and install ruby 1.9.3 without having to unmask the packages and associated use flag. It also means that we'll try to add the ruby19 flag, when possible, to new ebuilds as we add them. With ruby 1.8 no longer in maintenance starting June 2012 we can now work towards making ruby 1.9 the primary stable ruby implementation. At this point in time, though, we don't officially support running your whole system on ruby 1.9 yet.
Unless you take action yourself nothing will change for the moment. If you want to start installing your ruby packages also for ruby 1.9, then you must add
ruby19 to your
Note that this will only work in the testing tree for the moment. If you find bugs, please report them or drop by in