Thinking of the same setup myself- grad student, currently a school-provided Lenovo X201 (I swapped out an SSD in it to keep it serviceable) and a few year old custom built PC at home (Sandy Bridge i7 + GTX 570). However, graduating next month so I'll be turning in the Lenovo. My future employer will provide a new laptop (likely a 4-5lbs T-series Lenovo), but adding a nice ultraportable like the rMB makes some sense especially since I have an iPhone and iPad. Then, I might do a 27" 5k once Skylake rolls around.
I'm not super worried about workload on an ultraportable if I got an iMac eventually. My industry (hardware engineer) requires running CAD tools on server clusters most of the time, so heavy VNC and remote Linux use, nothing too demanding locally. Any local processing needs on the macs would just be for personal use and not work, but the larger the display the better if I need to VNC somewhere. I also use dropbox/icloud/etc. quite extensively, so not worried about keeping things in sync.
Caveat is a 15" or so retina display, instead of the 12", might be useful when actually working while traveling (back home to see family) though either would be a large upgrade my from my current laptop. Hopefully the work-provided Lenovo would suffice in that regard though not super excited about some of the specs they offer. Also, I'd like to upgrade my PC once Oculus is released, so still debating rMB + iMac + PC vs. rMB + rMBP 15" + PC or just rMB + [some USB-C external display + hub in the future] + PC. Don't really want to stock up on too many machines, but on the other hand I use facetime quite extensively so it would be nice to have that in more forms. I have the feeling the PC will be relegated to gaming and maybe as a NAS in a VM (in lieu of a dedicated airport extreme/timecapsule).
Waiting for the back to school sale to start before buying something, so a little more time to decide.