Curious as to how Apple has upstaged Microsoft in terms of market capitalisation, I plotted the market capitalisation of Apple against Microsoft between 1 June 2009 to 26 May 2010 (at monthly intervals except for the final data point).

Was it the iPad that helped Apple overtake Microsoft?
Seems like the gap between Microsoft and Apple remained rather constant at roughly US$60-80 billion until the announcement of the iPad by Steve Jobs on 27 Jan 2010 to a packed audience in the Yerba Buena Center for the Arts in San Francisco.
There was no looking back afterwards as the iPad was launched in April and sold its first million unit on May 3 after merely 28 days. This was less than half the 74 days it took the hugely successful iPhone to sell its first million units.
The gap between Microsoft and Apple closed rapidly until this week, when Apple finally overtook Microsoft in terms of market capitalisation.
At the rate it’s going, it won’t be long before Apple overtakes Exxon Mobil as the most valuable company in the US.