I've just ordered a SIM card from LeFrenchMobile; I do need voice too so a data-only product won't work.

I like these guys because everything is in English, and their support was very quick in responding to questions. I bought the SIM card for €22 (including shipping to the US) and it comes with €15 credit. I found a code that gave me another €5 voucher with the order. I will then top-up €40 online (can be done with my US credit card) which gives a 30% bonus, so I will have a total of €72 credit to start.

I'll then add the "XL Data Bundle" which is €39 for 300Mb data; this should be sufficient for my 5 days in Paris. I get free incoming calls (my US cell number will be forwarded to this French number at $0.069/min via Localphone) and can call within the EU and back to the US for €0.19/min (with €33 credit left over).

So it wasn't dirt cheap, I will spend $80 total but have good use of my phone and don't need to worry about finding/registering/activating a SIM upon arrival.

I will report back on my return how it worked out. This uses Bouygues' network and should be fine in central Paris.