I can see your dilemma, i am in university and i bought a macbook a month after it released and i was thinking about getting an imac now. But its true, you really cant use them both at once, and if you have two comps then you have to worry about making sure your all in sync (if you have an assignment on the imac and forgot and thought it was on the macbook, its a pain) .
What i did was i bought a nice 19inch widescreen display, and an apple bluetooth keyboard and mouse for my desk, when i come home i set down my macbook and plug it in to the external and it goes right to spanning dual screen mode, and its beautiful!
I really love it, cause it lets me have so much more screen space, and i get the comfort of a desktop at home with the external gear, and i upgraded to a 250gb HDD so i have everything i need with me all the time (and more on external HDDs).
It can feel like an imac with all the bluetooth gear, and it works great, and i really didnt NEED an imac, it would just be a really nice thing.. i think i would only buy one now if they give a big upgrade...
My vote is but a new monitor, a Mini-DVI to DVI and a bluetooth keyboard and mouse (or a wired set) and enjoy both worlds at a fraction of the price! (until you can no longer resist!)