Not sure these are actually Catalyst questions, but they are part of the workflow through Catalyst, so:
1) Are the "d" namespace attributes (such as "userLabel") compiled into the SWF when compiling in Builder?
<s:TextInput skinClass="components.UserTextField" text="Username" d:userLabel="UserTextField" x="43" y="82"/>
1a) If so, how can I access them in AS3?
1b) If not, what's the use? I know -- they carry over some useful data, but is there something we can do with them in Builder other than search and replace?
Any attribute that has a prefix (d:, th:, ai:, etc) is completely ignored by the compiler and is not accessible at runtime. They are called "private attributes".
They are not directly useful in Builder. Feel free to strip them out.
That's what I figured. Thanks for the confirmation, Adam.