I received my Digital Rebel two days ago. Rest assured, you _will_ carry it with you! Its weight is unbelievably low. May be you could get a nice bag for it as well. What strikes me, is how easy it is to use, given its powerful feature set. Compared to the S400 or S50 I believe it is significantly more light sensitive, which is a big deal if you are into available light photography (such as myself).
Best wishes, Jürgen
What keyboard ...
... is actually a pretty personal thing. It's almost like asking someone else what car you should buy. You have to remember that you are buying a musical instrument. What I like - sound, style of playing, feel - may be completely different from what you like.
If you want a keyboard that will support you in becoming a keyboard player, you wil want a different keyboard than someone who just wants a set of black-and-whites to smack on to input notes.
If you want a full-size (88-key) controller-only keyboard with action that closely resembles that of a real piano, try the Fatar SL880PRO Studiologic 88-Key Full-Size MIDI Controller (this was the keyboard you saw onstage at the MacWorld demo). It's not a synthesizer, just a keyboard, but you will not learn bad playing habits (especially good if you're planning on piano lessons, as the teacher will have a real piano. If you don't, it will become very very difficult for you quickly, as your practice keyboard and your lesson keyboard will not match in feel). That'll go for about $600. Pricey, but you're buying a real musical instrument from a good manufacturer, not a toy from Wal-Mart.
If you want to step down a bit, the Fatar TMK88 gives you 88 keys with a synth-type feel (not weighted). Again, if you want to know how it feels, you'll have to get it under your fingers first!
If you're in the "just want black-and-whites" camp, you can go with most of the Yamaha PSR keyboards; these are nothing to write home about as far as keyboard action or internal sounds are concerned, but they should be adequate as far as input devices go. How sturdy they are is another question; I have no personal experience with them, but they seem to be able to handle being whacked on by all the kids at Radio Shack.
There are of course lots of used synths, some fairly nice-sounding on their own, available at eBay or your local pawn shop.
I recommend cruising your local music store and talking to the folks in the keyboard department (the people fiddling with the keyboards, not the salespeople); most of them (us) are pretty friendly and willing to talk about what we like and don't like.
The used keyboard departments at larger music stores can also be a good place to pick up an older keyboard (Yamaha DX-7, Korg M1, etc.) that is both decent-sounding in itself and useful as a controller.
Hope this helps. If you want a detailed "how do I pick a keyboard" essay, see http://www.ibiblio.org/emusic-l/info-docs-FAQs/buying1st.html - good luck!
Yamaha has numerous portable keyboards to choose from, and some models have a USB connection for direct connection to the web.
If you're looking for a low dollar instrument, look at the PSR273 and PSR293. If you're willing to break the bank, look at the PSR3000 and PSR1500.
You can see a good selection at this link: http://www.andysmusiconline.com/Yamaha_Portable_Keyboards_s/3.htm
Good luck and have fun!