From my view, best to reveal it now and keep people from buying the XL (which I don't think was going to do spectacular anyway), rather than get 10 million people to buy it and then piss them all off by revealing the 3DS shortly afterwards.
Besides, I think the DSXL serves a purpose that the 3DS won't quite fulfill. It allows older people with bigger hands and maybe worse vision to play it. Like the Gameboy Micro and original DS, I think XL and 3DS can coexist well, at least for a time, because they cover somewhat different markets.