An interesting query.

VoIP is probably as secure as traditional telephony...

...and a lot more secure in most cases than your cellphone. Balance sufficient security against risks and benefits. If the effort of required to obtain the information in a VoIP call is greater than the intrinsic value of the information, VoIP could be considered "secure enough."

There's even a question as to how secure VoIP should be. For instance, there are legitimate concerns from the law enforcement community about whether advanced voice networks are "too secure" for court-sanctioned wiretapping.

At the end of the day, you should be a lot more worried about the guy with a glass on the wall than VoIP.