Online dating is regarded as a safe medium to meet other people. Thousands of people are meeting for the first time every day and incidents are few and far between. But that doesn't mean they don't exist.
The important thing is to never let your guard down and never give out too much personal information before meeting. Always be cautious and trust your instincts.
Most importantly, use separate transportation to meet your date at a public place the first time that you meet.
Remember to apply safety across the board. Just because a guy or gal says they are a "Christian" doesn't mean you can automatically assume they are OK. There is a case in the courts now of a woman who met a man through a Christian dating site and was sexually assaulted (News Article on Woman Assaulted - See Sources). Be alert always.
Interestingly enough, the greatest "danger" isn't from meeting someone who may assault you, but rather meeting someone who is married, yet posing as a single person.
Up to 30% of people using online dating services are married! (MSNBC, DateLists - see sources).
There are some easy signs, however, to separate those who are married from those who are single. Most married men will only give a cell phone number and you will mostly get their answering machine. Most don't post a photo (they don't want someone their spouse knows seeing them and spilling the beans). Most also won't give you their last name. (Staying Clear of Married Men - See sources).
Ultimately, though, it is a safe medium and tens of thousands of people have met their "lifelong" mate as a result.