Tips for coding a lengthy operation

Hey,
In our application, we have to send a SMS (and even emails) to hundreds of users.
Since this operation could be lengthy, what is the best approach to code it?
I mean, should I embed it into a thread (all the code will be in Grails service)?
Should I launch it via Ajax in order not to hijack the UI?
Your corrections and suggestions are more than welcome.
Thanks for help and time.

A couple of ways I might implement this are to either use quartz (there's a plugin) and schedule the task or I might just use a old school cron job. I'd say your main concern is not allowing this task to be kicked off more than once at a time by multiple users.