While the State of Texas does not have the right to secede from the union (as does any other state), It DOES have the right to add four more states within it's boundaries, including the State of Texas, for a total of five States. This is according to the (little known) language found in the Joint Resolution of the Congress of Texas, June 23, 1845 and can be found here: http://bit.ly/T3mxtA
Sub-dividing into additional states would also create eight new Senate seats in Congress which would help to balance the control of power that we see today.
For more information, please refer to this article:
http://bit.ly/T3muOt