I bit the bullet and updated my firmware to 3.0 a couple of weeks back. I did a bit of research though, and ensured I had all user data deleted before the attempt, and also noted the bit about not disconnecting it until told to do so - even if there seemed to be a significant wait. However everything went better than expected - in that it came out the other side actually working, and quite well.
My Edge 500 failed to acquire the GPS signal for the entire commute yesterday. It has been getting slower and slower to attain the GPS signal of late, particularly if you are moving. I had my philips light on yesterday morning,
On my way to work on Tuesday Morning, I glanced at the unit, and noted it was "updating Satellite software" (or similar) complete with progress bar graph. Had no idea it could do that, nor where it was getting this data from. I had a bad feeling about this, but it eventually finished, did a search for satellites, acquired, then resumed tracking the run - did not interrupt the track at all. Probably took about 3 or 4 minutes. Strange, but worked out okay.
A far as interference goes, I once had a horrible Kaiser Baas camera mounted on the bars. Kept on getting dodgy readings and tracks from the garmin, so one day I went to settings and checked the satellite signal strength (camera off) - all was good. As soon as the camera was switched on, the signal bars plummeted towards the bottom of the scale, restoring only when either moving the camera or switching it off.