Most certifications will get your foot in the door at big franchise gyms. For beginners to earn experience it's great, but if you want to deal with athletes (strength coach, for example) it's not going to get you there. The information presented may not be enough to be successful and get results.
You should be keeping up with research and new information if you want to be successful. There are a number of good courses that condense material into a few day seminars such as Charles Poliquin's programs (PICP and Biosignature), OPT (James Fitzgerald), and even Crossfit has some good courses (the specialty ones). There are a number of good books on the subject, but nothing replaces going to a good seminar. Staying up to date on the subject is essential because most people think all personal trainers are the same and give the same results, but you need to prove otherwise.
I've noticed that many people go into the field because of the low barrier for entry (certifications are easy to get) thinking that they can earn some quick cash. These people ruin the field for people that want to use a more intelligent and scientific approach. Of course, you also have too many trainers overstepping their legal boundaries, offering "nutritional consultations", "rehab", and even "sports medicine" to their clients. If you play by the rules, eventually there is a ceiling, and I think some people interested in the field are better served by training to become physical therapists, sports medicine physicians, dietitians, or other health care professions that deal with (somewhat) similar issues.
I decided not to go the personal trainer route because of the problems in the industry. I still enjoy reading about these topics and taking seminars, but I would never want to work in the same industry as some of the clowns that you find marketing themselves as "experts" in the industry.
If you're interested in the field, here are some good tips by Charles Poliquin, a very successful strength coach:
http://www.charlespoliquin.com/Arti...Want_to_Be_a_Successful_Personal_Trainer.aspx
http://us.cpoliquin.com/product_p/chp_podcast_4.htm
If any of you are familiar with John Broz, I think you will enjoy this blog. Hilarious comments by the man himself:
http://youtubebroz.tumblr.com/