From the Earth's position in the solar system, only the inner(inferior) planets, Mercury and Venus, can ever pass across the face of the sun. This event only occurs when the inner planet is at inferior conjunction(between Earth and sun), and when the planetary orbits line up correctly.
Mercury will transit the sun today May 7 2003, the first time since 1999. These transits occur on average 13 or 14 times each century. Observers will see the entire transit of Mercury from most of Europe, Africa and Asia. Japan, Australia and New Zealand will see only the beginning. Western Africa, eastern North America and eastern South America will see only the end of the transit. Transits of Mercury or Venus are only possible when the planet crosses the ecliptic- because Mercury's orbit is tilted 7 degrees to the ecliptic transits are not frequent. The transit will begin at approx 0500 UTC and end a few hours later. Only look if you have a specially designed solar filter fitted to your telescope or you may risk eye damage.