The macbook keyboard doesn't get to flex at all because it's fixed in metal.
Still, keyboard flex doesn't bother me at all in a keyboard, it's more a quality indicator of the construction. The key feel, the layout and the size are what counts.
The SL300 doesn't have the normal thinkpad keyboard layout, I think it's another keyboard altogether. Still, you can't assume it's bad. 
And about keyboard and thinkpad flex in the t400 (which is THE laptop to have now - all the options you would ever need like switchable graphics, humoungous battery life with the extended battery), please watch this video, to see a level of flex I didn't think was possible in a thinkpad.
http://www.youtube.com/watch?v=y8sOO-8LP4E
Lenovo now basically have two lines of separate levels of quality, there's the x200s, x200t and x300/301 that are a cut above all the rest. True thinkpads.
The price you pay for this quality is (like in the old days) steep.
An x301/x200t is double the price of a t400.
As an owner of a T400 (built December 29, 2008), I'm happy to report that my keyboard doesn't flex at all (it's a T6x KB), the plastics are sound and don't flex, creak, bounce, rattle, chatter, or anything else, the ThinkLight doesn't blind me, and the 14.1" LED
Samsung LCD is as clean, crisp, and bright as a person could want. The machine's integrated camera is another matter, as all subjects/objects have a yellowish tint, but I don't use the camera anyway. I also own a T60/p (built 5/2007) and the T400 is every bit the ThinkPad the T60/p is. If you can find a T400 at a good price, I'd not hesitate to buy it.
I'll also add that no matter what else is available in the marketplace, it's not a ThinkPad. Though some complain about Lenovo's build quality, shipping practices, etc., you'll still not find a better notebook at any price.
Cheers,
~rn