What are the relative literacy rates in for simplified and traditional characters?
More people can read simplified than traditional, by a significant margin. There are roughly 1.3 billion people mainland China where simplified has been the standard since the 50s and 60s. Taiwan and Hong Kong, the three places traditional is the standard, total about 32 million between them (23 million in the Taiwan/ROC, 7 million in Hong Kong, 2 million in Macau). There are a lot of people from mainland China and Chinese outside of countries where Chinese in the primary language who can also read traditional, but I have no clue on the numbers.

Most people can get by more or less when reading the other character set because there are a lot of very standard substitutions. It is however easier to read simplified if you only know traditional.

Example substitutions (some standard ones):
妈 马 语 诚 给 纽 约
媽 馬 語 誠 給 紐 約
(and some random ones):
龟 龙
龜 龍

Tools to convert between the two are readily available, although, as Ironfrost points out, these tools have a lot of problems with some of the 1:many conversion choices. Look at his comments below.

http://translate.google.com/#zh-CN/zh-TW/

http://translate.google.com/#zh-TW/zh-CH/