Never ask for a raise, unless you're willing to quit your job. Just look for positions at other companies.
by all means, pursue a raise at your current firm, but i think the assumption of the above comment is that many organizations fall into category (2) (see CyberFonic's comment), and i tend to agree. job-hopping is the more effective way to increase one's compensation in the long run.
edit: have a BATNA, basically.