I did this once. By the 3rd day of rehearsal, the PSM had negotiated with the producer to give me a proper Equity contract, because I was doing the work of an ASM. It worked out nicely for me...I got huge salary increase from the PA pay, overtime, my health weeks, and I got to run the deck for the entirety of the run. Of course, then we didn't have an actual PA for most of rehearsals, so I dabbled in both...making coffee and running errands while tracking props and calling the shops with modifications.
To answer Matthew's question about the psychology of doing this, of being willing to work "beneath" myself: I volunteered for the position when a PSM that I've worked with told me he was looking to fill the slot. It was a huge musical with lots of scenery, I had never worked on such a big show, and wanted a chance to see "how it was done". I had worked on musicals before, but none were so complicated. In all honesty, I didn't have the confidence in myself to think I could pull it off as an assistant. I was actually really excited to watch tech from the house, watch the PSM run tech and assist with his end of things. Since going Equity, I had always been an ASM, and therefore, felt a little removed from the FOH experience of tech. I saw it as a great learning opportunity, and a way to metaphorically get my feet wet on a huge show.