A quick funny story...
I was photographing a very old theater in Pasadena CA and doing the long exposure/make the people disappear thing. I was using the big Wisner 8x10.
When I developed the film, I noticed a head. One guy, curious about the camera and what I was doing, was hiding behind a pillar "spying" on me. You can see his face sharp and clear but his eyes are weird because of the blinks. It actually enhanced the shot.
In addition to people standing in place for periods of time, also take into account any other moving objects such as trees blowing in the wind, clouds, and the dreaded jet contours, if they happen to be in your scene. While moving clouds can be interesting, blurred trees may not be what you want. If you go the multiple exposure route, these moving objects might look a little weird (but I've done it, and it works).
Other thought -
Obviously it depends upon the lighting, but you can always pull or use ND filters or use a nice slow film.
If it were me, I'd test it myself varying all the parameters.
I would consider Les' suggestion on Multiple Exposures. He suggested the same thing to me a couple of weeks ago and I have been experimenting with it. My problem was similar to your except I wanted to leave a few ghosts. The multiple exposure method actually works remarkably well and has proved to be an easier way to control what I want in the frame.
David - I had something similar happen to me when I was working with some IR film in England.
I had just started playing with IR and was hedging my bets a lot. So I would stop all the way down for focus, put on the filter, rate the file as 12, and start bracketing like mad. I mean like 5 stop brackets here.
I was taking one shot of the ruins of an old abbey. In one corner I ended up with this ghostly image of a woman standing there and then turning away out of the frame. It must have been a pretty quick turn because I don't think I got below 1s for that exposure, but it lent a nice ghostly touch to the image.
Now if only she could have been in period costume....
I will say though that regarding "the curious" it can be useful to look for vantage points which aren't that noticable. I saw a great picture on PNet of Camden Market taken from a roof-top or window overlooking it. The long exposure caught thebustling of the market really well. And there were no "stand outs". Something to consider.
Actually, I'd bet that using both long exposure and multiple exposure would settle it, if you know your shutters well enough. I photographed a night scene in DC a couple of months ago, like a two minute exposure. I was standing on a sidewalk and had the problem of cars stopped at a stoplight right at front of me, but not all the time. So I counted seconds and broke up the exposure every time the light turned red and the cars piled in front of me. Worked well. I imagine it would be even easier with people...if the exposure is long enough, you can just close the shutter as soon as someone appears.
Nice avatar, by the way.
I took a couple of night exposures in Rome last summer. My favorite ones were of the Castel S'Angelo taken over the Bridge of the Angels. A high traffic area and I wanted to get down low. I wish I'd thought of multiple exposures but the end results weren't bad. I bracketed as well and the best shots were in the 5-15s range.
David I have to ask you, what the heck is your avatar? I've been going cross-eyed trying to figure it out! http://apug.org/forum/html/emoticons/wacko.gif