Developers developers developers developers, right?
Seriously though: yeah, Xbox has always been developer friendly, due to MS already having a lot of practice making development environments for Windows. They already had Visual Studio, teams working on compilers, teams working on Windows, teams working on D3D, etc. That's a pretty good position to be in to jump into the console game. BTW, every 360 devkit comes with a free professional VS license.
Both Nintendo and Sony have traditionally had really, really, really crappy development environments in comparison. Rough SDKs, rough tools, rough compilers, rough IDEs, etc... Post-PS3, Sony has been putting a lot of effort into improving things in this area.
Neither MS or Sony have said anything about XBLIG-type open development environments for the new consoles. Aside from homebrew hacks, XBLIG on the 360 is the only platform of this type still for home consoles.If they're willing to make the barriers to entry on their platform way lower and allow smaller teams to put their games on their system
Sony has made a lot of announcements about supporting independent developers, but you have to be aware that this is entirely different from XBLIG. This is about PSN/XBLA, which are not open platforms; these are only available to licensed developers. These gestures of "indie friendliness" are actually aimed at licensed development studios that are not publisher owned, not "indies" (people in garages).