At this time the strike is only scheduled for Thursday. Most likely, systems will be running Friday and the weekend but perhaps a bit slower than usual.
For TGV, Thalys etc check their websites or call for any cancellations.
Eurostar is reporting the following:
Between 20.00 CET on 21 May to 08.00 CET on 23 May 2008 there will be industrial action by general workers in France. Eurostar services between UK and France are expected to operate normally. However, it is likely to be severe disruption to Eurostar travellers with TGV connections to destinations in France. Also, other onward transport options in France – including the Metro/Underground and bus services – are also likely to be severely disrupted.
If Eurostar travellers with onward connections are able to change their travel plans they can exchange their tickets for alternative dates or obtain refunds by contacting their original point of sale. For more information visit Eurostar.com or contact the Ashford Contact Centre on 08705 186 186 from the UK or on 02/400.67.76 from Belgium.
http://www.eurostar.com/UK/uk/leisure/customer_care/questions_answers.jsp