Well, technically, the planets will never perfectly align. If you are looking at it from a 2-D perspective (from above) it seems that they would. However, if you look at a 3-D perspective, you will see that the planets are not all on the same level.
The planets when they do get close enough to be considering in alignment, there won't be too much to notice from it. If you are an astronomer or know what to look for, then you would have something interesting to look at in the sky in the weeks leading to and after the alignment. What you will be able to see is the planets grouped very close togehter.
In no way will this cause an eclipse. Mercury and Venus are way too small and too far away to cast a shadow on the Earth. The only thing that can cause an eclipse is the moon. That happens all the time already so you don't need an alignment to see that. You may have to travel to a different part of the world to really get a view of it though.
Some people have claimed that when the alignments of the planets happens, their increased gravity wreck havoc on Earth. That is not true. The alignment will not increase their gravity or combine the effects of their gravity. The gravity that Mars or Venus impact on Earth is insignificant and would be impossible to measure. The same goes for the planets that are even further away. The gravity of Jupiter (the planet with the most gravity in our solar system) has no effect on Earth. The closest Jupiter ever gets to Earth is 365 million miles. The effects of its gravity is weaker and weaker as you move away from it. After 365 million miles, there is almost nothing left to feel. So, the alignment of planets will not cause something bad to happen on Earth.
Here is an article that discusses this a bit.
Now, the 2012 thing. The Mayan calendar ends in 2012 or at least it is supposed to start a new cycle in 2012. Some people have interpreted this to mean that the world will end in 2012. This will pass like all of the previous end of the world prophecies in the past. The appointed time will come and go and the world will still be here. The end of the world prophecy crowd will move on and find another prophecy to worry about. Also, the next big planet alignment is in 2010.