MIVA.. hmm.. a language easy to learn but not customizable, and not secure. MIVA isn't necessarily restricted to shopping carts -- they can be used to make a dynamic clan webpae system (like the one at http://www.usimp.com/USIMP) - the whole page is coded in MIVA. Of course you don't get to see what its al about till you login.
Anyway, MIVA is flexible I admit.. but in terms of security, no. Databases are stored in a text fle (*.dbf) and usually not encrypted. Though they arn't stored online, one password can get you access to thousands of un-encrypted passwords.
Most MIVA sites I know are switching over to PHP, or ASP. Do the same also.. I see a very bleak future ahead for MIVA.
Miva is, indeed, doing something about their security issues. Miva 4.0 is just the beginning. I've worked with Miva since it was known as HTMLscript. I must say that it's a simple and very powerful scripting language.