I know I'll get a thousand "Depends on what you're trying to do" answers, but seriously, there really is no solid information about this online yet.  Here are my assumptions - I think they're similar for alot of people right now:

It is now October 2008.  I want to start writing an application for January 2009.  I am willing to use beta code and such but by January, I'd like a site that doesn't have 'strange' problems.  With that said, if a language is simply 10% slower than another, I don't care about those things as long as the issue is linear.  My main concern is developer productivity.
I'll be using Linux, Apache, MySQL for the application.
I want the power to do things like run scp and ftp client functions with stable libraries (I only picked those two because they're not web-related but at the same time represent pretty common network protocols that any larger app might use).  Technologies like OpenID and Oauth will be used as well.
Experienced web developers are readily available (i.e. I don't have to find people from financial companies and such).
Whatever the choice is is common and will be around for a while.
Here's a kicker.  I'd like to be able to use advanced presentation layer tools/languages similar to HAML, SASS.  I definitively want to use JQuery.
I will be creating a Facebook app and at some point doing things like dealing with SMS messages, iPhone apps, etc...

At this point, the choices for language are PHP (Cake,Symfony,Zend), Python (Django), Ruby (Merb).  I'm really between Django and Merb at this point mostly because everybody else seems to be going that way.  
Please don't put any technologies in here that aren't made for mainstream.  I know Merb is untested mostly, but their stated goal is a solid platform and it has alot of momentum behind it so I'm confident that it's workable.  Please don't answer with how great Perl is or .Net.
For Future References - these choices were already made:

Debian (Lenny) - For converting CPU cycles into something useful. Trac
0.11 - For Project Management Gliffy - For wireframes and such 
Google Docs/Apps - For documentation, hosted email, etc... 
Amazon ec2/S3 - For hosting, storage.

Cheers,
Adam
I have to preface this with my agreeing with Orion Edwards, choose the one your team is most familiar with.
However, I also have to note the curious lack of ASP.NET languages in your list. Not to provoke the great zealot army, but where's the beef? .NET is a stable, rapid development platform and the labor pool is growing daily. VB.NET and C# are transportable skill sets, and that can mean a lot when you're building a team of developers to work on a diverse set of tasks. .NET also allows you to separate your presentation layer from your backend code, like other languages, but also allows you to expose that backend code as web service for things like your iPhone and Facebook applications.
Take every suggestion with a grain of salt, and pick what suits the application best. Do your research, and design for function and not the zealots.
Disclaimer: Once a PHP, ColdFusion and Perl developer. Flex zealot, and Adobe lover. Now writing enterprise .NET applications. ;)
Don't forget Mono, which will let you run .NET under *nix. Not that I'm saying it will be perfect, just playing devil's advocate.
Framework/Language for new web 2.0 sites (2008 and 2009)