The settings anyone shares are practical in a sense, but there are also style choices being made here. You can get a way with a few approaches shared above, but they'll each give you a different look.
Let's say you're at the evening reception, shooting toasts or a first dance, in a standard hotel/resort ballroom: Shooting at lower ISO's (400) and smaller apertures (4-5.6) as suggested above is going to give a much more flashed look than shooting at ISO 1000 @ F2.8. Both will expose the subject okay thanks to ETTL, but one will look more 'snapshotty'. I do my best to balance flash with ambient when possible, and if not, at least bring in as much ambient as I can. In the average reception hall, with dim lights, I often start at ISO 800, 1/60th @ F2.8 on M, and the flash on ETTL, sometimes W/ the FEC on +1/3 or +2/3 if I need more power to bounce. If that's not cutting it, mabye even ISO 1000 or 1250.(I use a flip-it bounce diffuser, they rock.) With a 40D, ISO 800-1000 isn't a deal breaker and it requires less power from your flash. If the lights get dimmer for dancing, then it's not a big deal to drag the shutter a little and still freeze motion with the flash - though I'm really favoring the 85mm @ F2 myself, these days.
I would avoid doing formals at ISO1000+ or at F2.8, but I'm not opposed to those settings for anything else.
~Phil
