We are going to be in Paris next Saturday and were wondering what is the best way to see the night-time illumination of the beautiful buildings.

If this is a serious question then I'd suggest the bateau mouche on the Seine which starts at the Eiffel Tower and is a lovely way to see the sites at night. It cost 10 Euros each and has an audioguide in several languages.

Take the Bateau-Mouches just after sunset - Right Bank next to pont de l'Alma. They last about 75 minutes.

I don't know why anyone would think this wasn't a serious question. This will be our first time in Paris and we had heard that the illumination was beautiful. We also heard that there are bus tours at night in addition to the river tours. I just wanted to know what folks would suggest as the best way to see the illuminations.

lilla, you are correct that there are bus tours at night. I haven't taken one so can't comment on the content but they'd be hard pressed to surpass the river. They are 3 or 4 times the cost of the river cruises.

I haven't taken either so don't know for sure, although I wanted to my last visit but couldn't fit it in. I've done the cruise only in daytime. Neither is really that expensive for a special trip, although the bus tours are more like 30-45 euro, I believe, whereas the cruise about 8 euro. I think both would be nice, but I would do the bus tour myself because there are only so many buildings you can see from the Seine, and those bus tours go by a lot more places that are lit up (Opera, place Vendome, Louvre, Arc de Triomph, etc.). I'm not sure you can see the Arc from the river, maybe a little, but the river is rather low and you'll only see things right along the Seine. I"m sure that would be nice, though, just more limited.

Bâteaux-Mouches, definitely. It's not as though all of Paris is illuminated at night, but what there is can be seen best from the Bâteaux-Mouches. Pick up the ride at the north side of the Pont d'Alma. Your hotel can also arrange a ride.
I know nothing about illuminated bus tours, but I do know that there aren't that many things illuminated in Paris that aren't visible from the Seine, so I'd think the Bâteaux-Mouches tour would pretty much cover most of the illuminated stuff.

Here's the website for Bateaux Mouches:

http://www.bateaux-mouches.fr/English_index.htm

I'm not sure how familiar you are with them, but the boats have powerful lights mounted on them so they provide their own illumination of the buildings. (The lights are BRIGHT and I can't help thinking about how I might feel about this activity if I had happened to have been living along the river at the time someone first came up with this idea...)

Another company that we've used is Bateaux Parisiens, but it's been several years. They were docked practically in the shadow of the Eiffel Tower, just off Pont d'lena on Quai Branley.

Check www.cityrama.com which has a nighttime bus tour ($30) and a bus tour and river cruise package ($41). You can book at the website or visit their office at 4, Place des Pyramides (right by the Louvre - Metro Tulieries).

A boat cruise would be quite nice, but the smaller ones (Bateaux Parisiens) are much better than the Bateaux Mouches).

The bus tour is nice, too, if you don't mind going with a crowd.

Best bet is to make a list of the places you wish to see at night and handing it to a taxi driver...

Another vote for an evening cruise on one of the smaller boats. My perference is for the Vedettes du Pont Neuf: http://www.pontneuf.net/

Live commentary (French/English) instead of canned, multilingual tapes.

It's a fine question - you should do it on the river. The pace is slow, you can move around, and the perspective of being slightly below the buildings makes them all the much more lovely. Of course, I am tres irritated with the French and won't be visiting the lovely country for quite some time yet (heck, if we slow-rasted 11,000 of our citizens, the world's opinion would be firmly against the USA).
Have fun -