At Bitext we have been analyzing Twitter for several years now in several projects (among them, our NaturalOpinions Observatory for measuring the online reputation of the most popular Tablet models). One of the sources we use for getting data from Twitter is the well-known "firehose" which Twitter makes available by providing a small but representative percentage (currently, 1%) of everything that is written in this social media.
When we started out in late 2010, we used to download 900,000 tweets daily, and around 8% of them were in Spanish.
In late 2011 we were downloading almost three times the number of daily tweets (2,300,000), and the frequency of Spanish tweets had risen significantly surpassing 10% of total tweets.
Now, when 2012 is coming to an end, we are downloading from this firehose more than 4,000,000 tweets daily (in two years tweets have multiplied by 5), and what is most interesting to us, almost 15% of those tweets are in Spanish. The ratio of tweets in Spanish has almost doubled in only two years. This means that 65 million tweets in Spanish are written in Twitter nowadays. This is something worth considering.