I agree that there was a distinct push toward real independence (which would have been interesting!) but there were political forces in place from the beginning that pretty much ensured that Texas was going to join the Union once they declared independence. ;)
[edit] I agree Forty ;) The nine year experiment is an interesting one. All I'm saying is that there were just too many resources (natural and political) that Texas had to offer the Union. The US had its eye on Texas from the beginning, there was no way that Texas wasn't going to become a part of the US.