When Nintendo first unveiled the 3DS, we were told that the design shown wasn't final. Turns out that, yeah, it was pretty final.
Unlike the DS, which went through a fairly radical redesign between first being shown off and hitting retail shelves, according to Nintendo the appearance of the 3DS will remain largely unchanged between now and the handheld's release.
Asked by Wired whether the 3DS's outer appearance could change between now and whenever it's actually released, Nintendo's Hideki Konno replied, "You can take this as the final shape".
That's not to say there couldn't be a slight nip and tuck here or there - or that there won't be changes made under the hood - but it sounds like the current design of the 3DS is, by and large, how things will stay. At least until Nintendo releases a 3DS Micro, 3DS XL or 3DS Lite.