Within a few minutes after eating the soup and then the lettuce
salad/oil dressing at a wedding reception dinner, my wife felt faint
and then on the way home experienced one bout of projectile vomiting
and extreme weakness, the latter of which lasted for more than twelve
hours.  She had previously had no unusual food or drink across the day
and until the attack felt perfectly fine.  She had none of the
symptoms currently associated with the spinach/e.coli scare.  Is it
possible she experienced an allergic reaction to something in the
salad?  Does any kind of test exist to screen for possible chemical
allergens?

This sounds more like bacterial food poisoning than an allergy. 

This article may be of interest to you:

http://hcd2.bupa.co.uk/fact_sheets/html/food_allergy.html

Thank you very much for your quick response.  DW

Yes, I agree, caterers pre-prepare and warm things up.

Hopefully you have some pictures, the kids and grand kids will be interested.

I sincerely hope that you had both test driven each others cars, so
the evening was a minor disappointment - rather than ...