Question

I'm looking for a piece of software that fulfills the following requirements. I've exhausted my search capabilities.

Version control integration (SVN preferred)
Tickets support custom fields, including search and filter
Different types of tickets can go through different custom workflows
Support of multiple projects
User access controlled by roles assigned permissions per project and/or per workflow
Gantt chart
Time tracking aggregated by state in workflow per ticket, or generic 'activity' on ticket
View, Edit permissions per field, per role, per state in workflow
Ability to set whether a field is required per state in workflow
Release / burndown management
Continuous Integration integration (Maven preferred)
Eclipse / Mylyn Integration
Hard coded start / due dates, and expected start / finish dates based on resource availability (Nice to have)
Resource leveling (Nice to have)
What if scenarios - What would happen to the dates of Project A if I prioritized Project B (Nice to have)
Active Directory integration (Nice to have)

Redmine was really close, but the lack of ability to set permissions per project / workflow (tracker) is a show stopper.

Answer 1

I'm using Trac for this kind of job (see my answer here for details):

Version control integration (SVN preferred): Definitely. I'm using it with SVN, but other VCSs (e.g. git) are also supported.
Tickets support custom fields, including search and filter: Yes. And more, like master tickets/dependencies, tags, ...
Different types of tickets can go through different custom workflows: Custom workflows are possible. AFAIK this should be doable, though I've not explicitly verified the "different tickets use different workflows" part.
Support of multiple projects: I'm running it this way. For easy switching between them from the web frontend, there's a plugin available.
User access controlled by roles assigned permissions per project and/or per workflow: Per project definitively. Per workflow probably not, but there might be a solution for that as well.
Gantt chart: via plugin, yes.
Time tracking aggregated by state in workflow per ticket, or generic 'activity' on ticket: Again via a specific plugin, yes.
View, Edit permissions per field, per role, per state in workflow: Not sure what you mean here, but as everything is highly customizable, let's agree on a "maybe" :)
Ability to set whether a field is required per state in workflow: If I remember correctly, there was a way to define required/optional fields.
Release / burndown management: Yes, via the Burndown plugin
Continuous Integration integration (Maven preferred): Here I'm not sure, but I think I've seen a plugin for that as well.

For the other requirements, I'm not 100% sure. But as Trac is open source (it's written in Python), you can always add what you're missing. I did that myself a couple of times (I wrote e.g. the LogViewer Plugin, and integrated Trac with my local environment to do a lot of things via scripts). It's definitely worth a look.

As I've mentioned the plugins that often: Many of them are available via the Tac-Hacks site. Basically, Trac itself provides a solid core, and then you can customize it via plugins the way you need it.