How to choose a lens for Canon 7D video?

[ShutterbugJohan]

changing lenses (turn camera off so sensor is not electro-statically charged

I cannot speak for other manufacturers' products, but EOS Digital SLRs
with Canon CMOS image sensors do not carry electrostatic charges at any
time. Also, when DSLR image sensors are discussed, it's important to
remember that the surface of the sensor itself is never exposed to dust.

[ddi]

Thank you all for your advice. I think it's going to be either EFS 17-55mm f2.8 IS USM or EF 24-70mm f2.8 L USM as soon as I get the $ together (not yet sure which one to complement my 18-85mm). I was tempted to go for a couple of primes, but fear I will be too limited unless I have a full set and that frequent changes will give me too much dust trouble.

[Daniel Browning]

I only have
Canon EF-S 17-85mm f/4-5.6 IS USM lens. Would that lens be of any use
for video on 7D?

Yes.

What lens would you recommend for video (some of it in
low light) on 7D if I can only get one lens for now?

If it's for just video, you can get better results buying old manual lenses (e.g. Nikon), because they have much better manual focus features (rotation length, dampening, etc.). But if you need still and video together, you'll get the best results from prime lenses, especially L, because they tend to have better focus rings, less breathing, etc.