I believe Saudi Arabia was involved in 9/11. According to media reports, declassified documents describe the 9/11 Commission's interrogation of a diplomat who had ties to an individual within the terrorist network of the 9/11 attackers. I think it is highly probably low level government officials were involved in the 9/11 attacks.
9/11 was organized by a terrorist organization led by Osama Bin Ladin. This organization did not have any strong ties to any one country and any ties that Saudi Arabia had to 9/11 are flimsy and superficial in nature. So far no concrete evidence has surfaced to suggest any other ties.
Theories abound about who committed the terrorist attacks of September 11, 2001. Saudi Arabia can't conclusively be tied to these events because they have not claimed responsibility for them. As a result, I think that the perpetrators of this horrific event remain to be discovered and that we should keep searching for them.
No, there isn't solid evidence tying Saudi Arabia to 9/11. While a number of the hijackers were from the country, and several Al Qaeda leaders came from Saudi Arabia, there isn't any solid evidence that the country itself was involved. There is some circumstantial evidence of possible involvement by the Saudis, but that is not the same as having proof.