Ian,
I can confirm that DataAssist, Security Assist, Cookies ToolKit, Framework Builder, Validation ToolKit (CSS Form Builder), Dynamic Dropdowns and Image Resizer all work under CS5.5 on a Mac as expected
I will be using eCart shortly as well but I expect there to be no issues there either.
In general there should be no incompatibilities as DW CS5.5 is an evolutionary build, it is certainly more responsive than previous builds and the jQuery code hinting is great making it a nice upgrade for web professionals building dynamic sites.