Q1: "First, it is unclear to me whether, as an attorney, you always need a power of attorney from the inventors to file a U.S. patent application."
A1: You do not need an official power of attorney (i.e., a signed document) to file an application or prosecute it. You can file and prosecute an application entirely under "apparent authority." See MPEP 402 and 37 C.F.R. 1.34.
Q2: "But what about the usual case where the inventors have assigned their rights to a corporate entity and you (the attorney) represent the corporate entity (and have obtained a power of attorney from them)?"
Assignees can control prosecution of an application once they become of record. See 37. C.F.R. 3.71. Also 37 C.F.R. 1.32(a)(2) defines "power of attorney" as "a written document by which a principal authorizes one or more patent practitioners or joint inventors to act on his or her behalf[," and
(a)(3) defines "principal" as "either an applicant for patent . . . or an assignee of entire interest . . . ." IMO, these two provisions allow an assignee to effectively become the "applicant" for the purposes of the application, because it allows the assignee to control prosection directly or through a third party representative, independent of the inventors.
Q3: "in the case of PCT applications, the requirement to submit a power of attorney from each inventor was waived a while back . . . "
A3: I'll be honest and say that I am not sure about this one. I think the answer lies in how the PCT is filed. I know that generally, we will file a PCT application in the names of the inventors if an assignment of worldwide rights is not available at the time of the PCT filing, unless certain declarations can be made. I also beleive it is possible to designate who the applicant is by country (e.g., by including a statement such as "for all countries except the U.S., the applicant for this PCT application is the corporate assignee.") Of course, I have not done many PCT filings, so if anyone knows better please feel free to correct me.
Q4: "Third, in the case where you represent a corporate entity in filing/prosecuting a nonprovisional application, I have heard numerous attorneys state that it isn't necessary to obtain and record an assignment from the inventor(s) to the corporate entity if the inventors were employees of the corporate entity at the time of invention since generally it will be the case in the US that the employment agreement will operate to assign the rights of anything invented by the employee (within the scope of his/her employment) to the corporate entity."
A4: 37. C.F.R 111 allows an "application for patent" to be made "or authorized to be made" by the inventors. In many instances, an employment agreement will specify that an employer owns any inventions developed by an employee while working for the employer (or developed on the employers behalf). In addition, it is often the case that the employment agreement will also specify that the employee authorizes the employer to file patent applications drawn to such inventions in accordance with U.S. law. In those circumstances, I believe that the employer could authorize outside counsel top draft an application drawn to an invention developed by the employee, and file such application in the USPTO in the name of the employee ("in accordance with U.S. law). That said, I would never assume anything about an employment agreement.