Update 5 June 2012: Over the next few hours the planet Venus will transit the Sun. This will be visible from many parts of the Earth. Remember to observe with safe solar filters. You can also view the whole thing online via LCOGT, The Royal Astronomical Society or NASA Edge.
Every time there is a solar eclipse you will find astronomers warning you to never look directly at the Sun. Even more importantly, you should not look at the Sun through a telescope unless you have a professional solar filter that covers the front of the telescope. Why?
The Sun is very bright and by focussing the light onto the back of your eye (the retina) with or without a telescope, you are putting a lot of energy (both optical light and infra-red) onto a tiny area. At some point in your life you may have tried to set paper on fire using a magnifying glass, so just think about that being done to the back of your eye. It isn't nice. Even more scarily is the fact that the retina of your eye does not have pain receptors, so you will not even feel the damage being done. It may not even become apparent until later.
A grape at the focus of a small telescope while looking at the Sun unprotected. CREDIT: Stuart/Megan
Not only do you blind yourself, but you may damage the optics of your telescope. So, remember folks, NEVER LOOK DIRECTLY AT THE SUN. If you want to observe the Sun, check out the SOHO website or find someone with a solar telescope. You have been warned.