I do a lot of video shoots, and the cell phone problem has been getting worse year by year. Cell phones definately can cause interference in any audio setup, even completely balanced setups because the mic itself can be effected even before the signal starts going down the balanced pipes. Although unbalanced systems (like more stealth systems) are even more prone to it because the interference can be picked up anywhere in the chain, whereas a good balanced system will have some ability to cancel it if it tries to enter anywhere downstream from the mics themselves. Not 100% sure I've got that correct, but that has been my personal experience with audio capped during video shoots.
One thing I don't think I saw mentioned above is that the TYPE of cell phone really makes a HUGE difference. GSM phones are really, really bad because they "talk" to the system even when not making or receiving a call (or text message). If you have a GSM phone and leave it next to your speakers at work, it'll cause iterference even when absolutely NOTHING is happening re messages or calls. With my old-system TDMA phone, I only got that type of interference when actually getting a call or message. The REALLY BAD news is that everything in the states is moving towards GSM and away from TDMA, so when combined with the proliferation of cell phones in general, the use of GSM phones is growing even more than that. This is also part of the reason why this has been a much bigger problem in other countries who were using non-TDMA systems longer than us.
Note: I might have confused TDMA with CDMA, but the point is that cell phones definately cause interference, and some types of cell service are worse than others. It is a well-known fact in the video industry that plenty of recorded dialog for video and movies have been completely fucked up by this causing numberous hours of re-recording or touch-ups in post.
I would definately turn off your phone while taping as you are the most likely person to be the closest to your rig more than anyone else. Just my two cents...