dvdexter, I just got back from Paris-BRU yesterday, here's how it works:

From Brussels airport, there is no direct train to Paris, you have to transfer at Bruxelles-Midi station (unless you want to stop over Brussels downtown, then you go to Central Station)
You will need two RT train tkts: one for InterCity Express, Airport->Bruxelles Midi, which takes 20-30 minutes one way, and one for Thalys Bruxelles->Midi to Paris Gare du Nord, which takes 1 hour 30 minutes one way
(make sure you tell the tkt agent first or second class)
If you know exactly the return date, get the less expensive but restrictive 'loisirs' fare.

From Bru to Paris, I was not in a hurry, so I just got on the next available train.
From Paris to BRU, I took the earliest train at 6:25a.m., arrived Bruxelles-Midi at around 8:05a.m., disembarked, then immediately hopped on the Airport Express at 8:18a.m., arrived at the airport at 8:48a.m., at Delta's check-in counter about 9:00a.m. for my 10:50 flight to JFK.

There were long lines everywhere. So allow yourself plenty of time. I made it to the gate at 10:05a.m.

(It may seem a bit complicated, but is not at all.)

I still have the fare/time schedule booklet for Thalys. If you need more info, please email me. Would be more than happy to answer any questions to my best knowledge for you.

[email protected]