I'm sorry to be such a bore, but I really do think the flight is too early in the morning to get the first train from Cascais, and get to Lisbon Airport by bus two hours before for check-in.
The 05:30 gets into Cais de Sodre at 06:10.
The Aerobus only starts from there at 06:55
There is Nº. 44 departure at 06:23 which might get you into the airport at 07:00.
The schedule shows a transit time of about 36 minutes. It is borderline.
However, to really be sure, I would recommend a taxi from Cais de Sodre to the Airport. (Especially, it you have bags.)
Note that once the Linha Vermelha Metro link from Gare Oriente to the Airport opens, hopefully towards the end of 2011, this advice may change.
