The question Do you actively think about security when coding? asks about security mindset while programming.

Obviously, a developer does need to think about security while coding — SQL injection, password security, etc.

However, as far as the real, fully-formed security, especially the tricky problems that may not be immediately obvious, should I be concerned with tackling these throughout the development process, or should it be a step of its own in later development?

I was listening to a podcast on Security Now and they mentioned about how a lot of the of the security problems found in Flash were because when Flash was first developed it wasn't built with security in mind (because it didn't need to) — therefore Flash has major security flaws at its core.

I know that no one would want to actively disagree with "think security first" as a best practice, but many companies do not follow best practices. So, what is the correct approach to balance between needing to get the product done and developing it securely?