I am a sole trader who is about to incorporate a company.
My business cards still have my personal name, although I plan to
migrate all my trading activities through the newly formed limited
company.
Can it be argued that I am still trading as a sole trader because my
business cards are the same ?
In other words, what is the "cut off" point between the business
trading as a ST or through a Limited Company ?

I think you will find its the date of incorporation. As from that date
you become a director of the company and you are therefore acting as
the company not the individual any more regardless of what it says on
your card. However, as an incorporated company you must make it clear
to your clients that you are incorporated so you could be decieving
them by using old cards as you now have limited liability under
incorporation.

Check out a book called running a limited company from Jordan.com its
very good and simple for people like me!

Actually I thinks its jordans.com