Some wise words above, obviously from experience

One thing that isn't clear yet but is the most important question: Why do they want the curtains cleaned? If you don't know why, you can't match a solution to the problem

If it's for visual appearance of the linings, you might be in trouble because dry cleaning won't really fix the yellowing from the nicotine. It'll improve it a bit, there will be a lot removed, but it won't look all evenly "clean" and light coloured again.
Just dealing with dust & dead flies etc is simple, you can clean in-situ. vac, spray with detergent free solution & towel off.
Smell - no problem, vac, spray & wipe as above adding deodoriser into your solution (i'd probably do that as standard anyway).
Maybe the top level service would be to have them dry cleaned off-site, hang them back up and then treat with deodoriser to finish off.
Good shout about flame retardant treatment, you'll need to ask if they are obliged to have this done. If so it needs to be the last process, if i remember right a flame retardant isn't affected by dry cleaning but water based products will remove it.
get yourself a hunter turbo vac, they're brilliant
