I recently received an email from a Documentum developer asking about what technologies he should learn for the future, especially for job security. Although I know Documentum technology well, I do not necessarily have a gauge on the job market. Other bloggers like Virginia Backaitis tend to have a good pulse on the market as well as the development community.
If I was a betting man, I would wager some money on learning ExtJS. I wouldnt double down on it though.