Update: This is PhotoShop 12.0.4 on Mac OS 10.6.8. Highlighting is stopped by certain punctuation such as semicolons, colons, periods. Highlighting is (understandably) not stopped by dashes. If you have two-words both will be highlighted when double clicking *including* any spaces after those two words
I can reproduce the behavior you describe - but I don't think this is a new behavior in Photoshop CS5 (I checked Photoshop CS3) or unique to Photoshop (Illustrator has the same behavior). Interestingly, InDesign doesn't have this behavior. We'll look into it. Thanks for reporting it.