The Rails Bugmash 2010 is well under way. I've compiled a few notes about what I've seen.

will_paginate

The current version of the gem (v2.3.12) doesn't work, as it relies on the now-removed scope method. It looks like there's a fork out there which resolves the issue, but it hasn't been merged into master yet.

Formtastic

The current version of the gem (v0.9.7) seems to work just fine. All you need is an initializer to set the gem up and load it into rails:

require 'formtastic'

# from formtastic's rails/init.rb
ActionView::Base.send :include, Formtastic::SemanticFormHelper

validation_reflection

You can use validation_reflection (v.0.3.1) with Formtastic by simply requiring it in the initializer:

require 'formtastic'

# from formtastic's rails/init.rb
ActionView::Base.send :include, Formtastic::SemanticFormHelper

require 'validation_reflection'

Authlogic

Not sure about the working state of the gem yet, but the 2.x style generator is clearly not working. I've forked Paul Berry's list of Rails 3 generators and added a simple user session generator. Edit: Actually, the gem doesn't work either. Seems to rely on a before_persisting callback which isn't there anymore.
Edit II: The before_persisting callback is actually built by Authlogic. It seems the Authlogic::Session::Base module isn't loading callbacks like it should.

RSpec

This is also a no-go in Rails 3. David Chelimsky, being the always-on-top-of-it developer, is already planning for RSpec 2.0 as a working partner for Rails 3. No sign of it yet in the github repo though. If you're interesting in playing around anyway, you can find some RSpec generators at the link above.

Bundler

The new gem bundler is indeed pretty sexy. Unfortunately, with all that great work it does, it doesn't actually load the gems into your Rails environment. Neither do any of the config files (boot.rb, application.rb, etc etc). David Trasbo had some luck adding Bundler.require_env to the end of the environment.rb file bundler creates, though he's run into load-order troubles. José Valim recommends placing the line at the end of application.rb to ensure the Rails core is loaded first. This takes care of loading all the gems, so you can skip the initializers above.