I just happened to get a forward which says that TajMahal was an ancient Siva temple which was built even before the times of Shah Jahan. I thought it was one of those usual hoax mails. But out of curiosity, I did a little bit of googling and I felt that its a serious discussion which is still going on. Some data that were stated was interesting..
* How Taj Mahal was derived from 'Tejo Mahalaya'(Shiva's Palace)
* The writings of some visitors from Europe which indicates that Taj Mahal was there even before the time it was supposed to be built.
* The use of captured temples and mansions as a burial place for dead courtiers and royalty was a common practice among Muslim rulers
I don't know how to validate this information. But yes, I am extremely curious to know the truth. I wish I could open one of those 'supposedly' sealed rooms of the Taj Mahal, and have a peep inside.

Courtesy: http://www.bbc.co.uk/dna/h2g2/A5220