The 360 is underachieving simply by Microsoft's own standards. At the beginning of the generation, Microsoft was undeniably certain that the 360 would be the market leader and turn an overall profit, but that plan rapidly went awry as the Wii took the gaming community by storm and instead, the 360 is now a distant second.
The 360 may be up year over year, but that isn't really saying much considering that its sales at this point in time a year ago were nothing to write home about to begin with. The fact that it's outsold the Xbox doesn't mean a lot either considering that after selling only 24 million, it really had nowhere to go BUT up, especially when that the 360 had a year on the market to itself.
Comparing it to the PS3 to measure its success is also flawed considering how poorly the PS3 is selling, and it actually makes the 360 look bad when it's only outselling it by 10-15K/wk worldwide and when it's only outsold it by about 3 million since the PS3 launched.
Even in software, it probably isn't where Microsoft wants it to be. The best-selling software of the generation is on Nintendo's platforms. Microsoft was also supposed to be getting PS2-level support from third party devs, but instead, it gets roughly the same support as the PS3 it's supposedly destroying.
As far as 360 sales exploding in 2010/2011 are concerned, forget it. The 360 simply doesn't have that kind of appeal. It still appeals mostly to core and hardcore gamers, most of whom will already own a console by then.
The 360 only looks good when you ignore the Wii and exaggerate the slim margin it has over the PS3.
Consoles owned: Saturn, Dreamcast, PS1, PS2, PSP, DS, PS3
Another thing to look at(and the OP mentioned this somewhat) is that for all of the hardware, accessories, Xbox Live subscriptions, and software moved, the 360 has made virtually no money whatsoever.
Consoles owned: Saturn, Dreamcast, PS1, PS2, PSP, DS, PS3