Only if you come from one of these countries:

http://travel.state.gov/visa/temp/without/without_1990.html#...

or a few miscellaneous others (Canada, Mexico, Bermuda, etc).

Ah, thanks. I am embarrassingly un-travelled.

No, you're actually right. The comment above links to a list of visa-waiver countries, whose citizens automatically get a 3-month (IIRC) visa on entry to the US. Tourist visas are NOT limited to these nations, it just means that if you're from another country (like me) you need to get in before you leave to the US.

OP's original statement that he requires a business visa to attend & speak in a conference is just ridiculous.