In many states in the US, there is legislation pending to require that individuals who appear in court as digital or computer forensics experts also be qualified as Private Investigators or similar state-recognized para-law-enforcement designation. Is the field of digital forensics sufficiently particular to warrant its own certifications? Or rather, is digital forensic analysis (not technical evidence acquisition but its analysis) merely another specialization within a broader category of evidentiary expertise?

I am sure, this is the wrong way! It is a great difference between doing a very specific forensic report and doing a private investigation.

I am of course quite far from the US legislative problems, but question is very important. I hope nobody here will use this idea as a useful example!
I have read a very huge discussion about problem of licensing US computer forensic experts as "PI" at "ForensicFocus".
You can find some conclusions there. Good luck, thread is very long

I agree. Though I am registered as a private investigator in the state of TX (until this October), I believe that the field of digital forensic analysis has unique elements that do not fall under traditional evidentiary processes. The efforts by many states to require dual certification seems to be more and effort by private investigators to ensure their market position than genuinely in the best interest of due process.

Main problem is in understanding of two different terms. I have not any exact definitions (and it will be fine, if somebody will be able to find some official definitions), but I understand next terms as:
Investigator - a person who investigate a (criminal) case. The investigation is process of collection and juristic interpretation of the evidence.
Forensic examination is the process, where for explanation and understanding of complicated phenomenons a very specific scietific examination is necessary.
Forensic expert (based on Czech, and as I know based on other European laws) is not allowed to make any juristic conclusions, he is allowed to do only a technical explanations (what is in total opposite of duties of the investigator).