First of all, I should announce that I don't know AS/400, I only know it's an IBM server with OS in the same name. (am I right?)
AS/400 job is not as popular as java and ASP, obviously. But AS/400 is mostly being used by big companies like bank, insurance.
If you are good at AS/400, that's good. In the tor.jobs newsgroup, you can easily find tons of wanting ads daily. But the point is, as AS/400 handles the most important data of a company, the employers may require a Clearence Certificate. Unless you're PR or citizen, it seems not that possible to hire an AS/400 engineer overseas.
So I believe you'd better apply as immigrant first.