You'll need to go over the new features of CS4 and then CS5 to cover everything. Lynda.com or kelbytraining.com have great video trainings that systematically highlight the new features. A month's subscription to Lynda.com costs $25 (you don't really need the premium subscription) and it is well worth the money.
One of the big changes thats throwing, me for instance, is that I can no longer simply cmd click on a eyedropper target on skin then cmd 1, 2, 3 on the RGB curves layers to set points to adjust colour.
In the Adjustments Panel it’s alt-2 (composite) and alt-3 (red), alt-4 (green), alt-5 (blue) to select the individual channels now.
And command-shift-click to set the points on the R-, G- and B-curves.
You may want to consider setting the Adjustments Panel to »Auto Select Targeted Adjustment Tool«.
One can still trigger the old modal dialog via a Script, but it might be better to just get accustomed to the Adjustments Panel.
You may be able to download the Help as pdf for CS4 and CS5, their respective »What’s new?«-sections should provide an overview of the accumulated new features.
Again, Christoph and I think alike, I searched for those, and he posted in between:
What switchers from CS3 have more trouble getting accustomed to is the Tabbed interface, and the Open-GL animated zoom/pan tools.
Many disable scrubby zoom, because they are used to draw a marquee to zoom, but both have merits.
And about the Tabbed interface and application frame/bar, that is especially useful when one use displays multiple documents.
Some disable tabs from the preferences: In preferences- > Interface turn off "open images as tabs" and "Enable Floating Document Window Docking". (But I recommend to try them, and see where they can fit in one's workflow)