I've enjoyed all of your responses. However, some seemed to over-react a bit. After all, I was asking questions about common concerns and giving an opportunity for the experts to respond to or debunk those concerns.
Let me also clarify that I NEVER meant to imply that any hybrid herps have been released or escaped to the wild. I have no such info. The wild hybrids (mostly turtles) that some have mentioned probably resulted from released non-native normals breeding with natives.
Anyway, I've been thinking about this alot since my original post. I have just about convinced myself that "hybrids" pose a MUCH LESS SERIOUS RISK than "normals" of becoming harmfully invasive. Below is a summary of some factors comparing potential invasive risk of hybrid vs non-native normal herps. These are just my personal assumptions. Please critique, debunk, or add to this list at your leisure.
RISKS OF INTRODUCTION TO THE WILD:
RELEASE BY OVERWHELMED HERP OWNERS: Most commonly due to excessive size, feeding difficulty or aggressiveness. Normals are more likely to be released for these reasons. From what I've read on this forum, hybrids tend to be easier to feed, less aggressive and somewhat smaller (at least the boids -didn't HEH say that?) than their source species.
ESCAPE: So far at least, hybrids tend to be more high-end and are therefore probably kept by more experienced and responsible collectors. Hence, normals would be more likely to escape.
DELIBERATE RELEASE BY EXPERIMENTAL IDIOTS: Do you think any herpers are stupid enough to intentionally attempt to establish a wild population? I would bet money that some novices would be that dumb. However, it seems that breeding hybrids requires considerable knowledge and experience. Hopefully, that means that hybrid breeders (the only ones with sufficient numbers of animals to have any potential impact) would be very unlikely to attempt such a venture.
SURVIVABILITY FACTORS:
BENEFICIAL COLORATION/PATTERN: The hybrids I've seen are pretty extreme and not likely to last long in the wild. After all, a neon orange snake would have a hard time hiding from predators or prey.
ADAPTABILITY: This is the only factor (IMO) that suggest hybrids could pose a greater risk of becoming invasive. Most captives would adapt poorly to the wild. Especially if released into a difficult climate. However, the often mentioned "Hybrid Vigor" MAY tend to give hybrids an edge at wild survival.
GENETIC IMPACT: If established, non-native normals would tend to retain their genetic make-up. It seems to me that they would be unlikely to hybridize with natives. Instead they would compete directly against natives. However, hybrids would likely breed back with one, both or all of their native source species (assuming one or more is native). I would expect that their genetic impact would be diluted with each successive generation.