Looking for a hotel in Paris that is less than one half hour by cab to Orly airport.

The reason you aren't getting many replies is that there is not really anywhere in Paris where you can guarantee to get to Orly by taxi in less than 30 minutes.
For example, from Place du Panthéon, which is in the south of the city, to Orly airport, according to www.viamichelin.com (which is known to be conservative in its estimates):

Time: 00h24 with 00h06 on motorways
Distance: 14 km with 7.5 km on motorways
Traffic Impact: +23 mins to 13:15

I.e. if you did it now (13:15) it would take 24+23=47 minutes. And that is not in the rush hour! and leaves no contingency.
From Montparnasse (further south in the city):

Time: 00h20 with 00h06 on motorways
Distance: 14 km with 8 km on motorways

But, that cannot be guaranteed, there could be traffic or other delays.
How far in advance to get to Orly depends on where you are going to. European flights have shorter check in than intercontinental; and budget airlines have longer times than "full service" airlines. Your airline should inform you, and it should be on your booking confirmation. This should say when check-in will close.

Staying around Denfert-Rochereau would probably get you there in under 30 minutes. There is the direct Orlybus that leaves from there to the airport and their schedule says 25 to 35 minutes, so if a bus can do it, a taxi should too, especially at that hour. The first Orly bus is at 5:35 am and costs about 7.50 euro per person. That would get you there shortly after 6, which may or may not be OK. I haven't flown out of Orly, so no first hand knowledge of the situation there.
The area around Denfert-Rochereau is perfectly fine and there are plenty of hotels nearby.

Try Hotel du Parc Montsouris on the southern edge of the city with the quickest access to the autoroute to Orly at the Porte de Gentilly. Otherwise, there are a lot of chains at the Porte d'Orleans right over the boulevard Peripherique that house a lot of group tours. Porte d'Orleans is the southern edge of town. These all have quick access to the freeway to Orly--but no guarantee on timing.

This one is charming; have to eat outside the neighborhood as it's residential and not commercial. View on the park.
http://www.hotel-parc-montsouris.com/en/

The advice to stay on the southern side of the city (as in the last two posts) is good. What day of the week is your 8 am flight?

We fly on Thursday, October 1.

ok, so a weekday/workday. So that doesn't help any -- i.e. the roads aren't clear as if it were Sunday morning heading down there. You will mostly be going against traffic though so it still should be okay even though it is rush hour (you'll be going very early if your flight leaves at 8 am anyway).

Thanks to everyone for the advice. I found some where in the area you recommended. They say it should only be 20 minutes or so by cab to the airport. We'll just have to hope that there is not another cab strike while we're there.

Hi from Wisconsin,
We spent the last night of our France trip at the airport sleeping at the Ibis. We have never stayed at an Ibis...we have our reasons. But it was compact, safe, clean, and quiet. And it was so nice to have a leisurely morning, sleep in late, grab a bite to eat, then grab our luggage and walk over to the boarding gate. Food offerings were not great. Poor airport food. Especially after eating all that great food in France.

All for under $100. Well under $100.

If you want to spend the night in Paris central. Go to the airport, check in, wash up and head into Paris vie bus or metro or the train. Do our last night sight seeing and head back out to the airport.

wayne iNWI