The lenses used by professional sports photographers (who need to get great telephoto shots) have focal lengths of 500mm to 1000mm, or even greater. (A standard 'point & shoot' camera will have a lens which has a focal length of around 40mm to 50mm). The closest you'll get with a camera that doesn't cost megabucks is something like this, which has a zoom lens going up to 720mm:
http://www.tesco.com/direct/fuji-finepix-s4500s4900-digital-camera-black/413-7120.prd?pageLevel=&skuId=413-7120
If you'd prefer something that you can easily slip into a pocket or purse, the Olympus SZ-14 (which has a maximum focal length of 600mm and dual image stabilisation to reduce camera shake) could well fit the bill:
http://www.olympus.co.uk/site/en/c/cameras/digital_cameras/traveller/sz_14/index.html
(It's a little over £100 from most outlets. Tesco change their price on it almost daily, varying from £99 to £139 but with £109 being the most commonly charged price).
Given that conference halls aren't usually particularly well-lit, and that flash units are useless over any great distance, image-stabilisation (to avoid the camera shake that often occurs when using long lenses) should be high on your list of priorities.
Chris
PS: I've got an Olympus SZ-14 and I can highly recommend it.