Arbrister knows he is vulnerable. Take a look at the last election analysis for his district:
http://www.capitol.state.tx.us/par_rpts/current/senate/dist18/r8.htm
His district went 62.4% for Cornyn for US Senator, and 66.5% for Perry for Governor. Of the four US Represenatives that straddle his district, 3 Republicans (DeLay, Paul, Carter) got at least 66% of the vote.
Of the rest, I thought Whitmire might be vulnerable:
http://www.capitol.state.tx.us/par_rpts/current/senate/dist15/r8.htm
But, while 3 out of the 6 US represenatives that straddle his district are Republicans, the largest block of votes went to Jackson-Lee.
Only 44.5% voted for Cornyn, and 46.9% for Perry. So, it's not likely that any Texas Senate seats will change parties in the near future, unless the backlash gets Armbrister. And Armbrister is a good guy (he authored SB501 in the regular session).