Oh I think so but I probably can't do a very good job of explaining it this morning (or ever). Tex-Mex was originally a peasant cuisine which grew up in the border region of south Texas characterized, as Bushie put it on another thread, by melted cheese and beef gravies. There are some common dishes and spices, such as fajitas, which were originally served in south Texas and northern Mexico, and cumin and chiles.
When I think of 'southwestern' cuisine I think of something more recent, trendy and upscale, such as the PBS series by Dallas chef Stephan Pyles from a few years ago. SW cuisine would be more likely to include borracho beans or charro beans or even black beans rather than refritos, or no beans at all, and more likely to use grilling or smoking as a cooking technique. The influences are drawn from Southwestern states like CA, AZ, CO and NM and even west Texas as opposed to south Texas.
See this thread for more on Tex-Mex: http://www.roadfood.com/Forums/topic.asp?TOPIC_ID=13572.
Prior to the publication of Diana Kennedy's The 'Cuisines of Mexico' in 1973 Tex-Mex was simply referred to as Mexican.
That's the way I think of it, anyway. I hope some more will chime in with their explanations.