That is exactly what I am planning on doing. To be fair though, even maxing out with the 3G 64GB, the cost is something I can easily afford.
Of course, all of this depends on your individual needs. I never do anything more complicated than editing/burning movies with iMovie. However, what my general computing needs lack in intensity they more than make up for in frequency and portability. Up until now, that has meant a 13" MacBook Pro that travels with me on pretty much a daily basis.
Soon, my iPad will take over as my most mobile computing device (ipod touch aside), as I believe it will handle the vast majority of my daily tasks in a smaller, lighter and more compelling form factor. My MacBook Pro will become my mobile desktop for long business trips (3+ weeks) and the rare day when I will need more outside of the house than my iPad.
Answer:
MacBook Pro = Sometimes Device
iPad = Daily Device
iPod Touch = Never Leave My Pocket Device