It was recently agreed with ICAAN, that .su domain will not be phase out.
There is strong resistance to .su by current russian domainers, because it erodes the vaule of their .ru domains. Further, there is talk of adding national domain in russian language, which will add more names to the market, and again erode the value of .ru as well as of .su domains.
Starting from december 2007, the price for .su was lowered from usd 100 to approx. usd 20, which resulted in increase of regged domains from 11,000 as of 01.12.2007 to 31,000 as of today (for comparison - .ru has 1,160,000 domains regged).
Anyone can register .su as well as .ru, there are no limitations. The price is the same for both residents and non residents.
Currently, there is a difficult procedure of selling the name - you need to send a letter to your register if you want to transfer the name to another user (same procedure as with .ru domains). It is expected that this procedure will be eased and made more like normal .com selling procedure, i.e. transfer to be made electronically. However, it is not know when this will be done.
The largest registrar is nic.ru (I use it), you can also use reg.ru or r01.ru. There are several other (10-20) official registrators, and many resellers.
I own around 50 of .su domains, some nice russian words, some english ones -line fruit related domains, i.e.blackberry.su for project of planting blackberries in some russian agriculture regions, .
After I got the names I liked, I stopped bying. In my opinion, the names are great for projects, i.e. I regged adultdating.su (there is a project already), kissing.su, colt.su, LLLL.su, memo.su, modeling.su, steak.su, tone.su, etc, but may be not as good for reselling. Consider this investment as highly risky, and do not allocate more than 5% of your budget to it.
Summarising - there are good chances now to get really nice names for you projects. There are high risks regarding the future value of these domains on one hand and a possibility that 150 million people of Russia + 100 million from former soviet union republics will use this extention on another. By the way, light.su is owned by IKEA.