Year-to-Date Top Five
In comments sent to the press, NPD Group analyst, Anita Frazier, said that Guitar Hero III and Rock Band (combining sales across all platforms) are in the top 10 software titles for the year with 2.5 million and 1.3 million units respectively. This leaves open the question of what other titles are the top sellers, year-to-date.
Provided we group sales of multiplatform games together, we estimate that the following are probably the top five selling titles for the year. Please note that the NPD Group did not release this list, but we have pieced it together from existing data.

(It is possible, although we think improbable, that a cross-platform game has sold well enough in 2008 to rank in the top 5 despite no single version of that game appearing in any top 10 this year.)
Grand Theft Auto IV was expected to be a top-selling game for the year, and so far it is on track to do so. However, it is worth noting that three single-platform titles – specifically on the Wii – are in the top five for the year, all published by Nintendo. Moreover, the top-selling version of Guitar Hero III for the year is for the Nintendo Wii.
As a measure of how robust software sales are in 2008, consider that three of these five titles would have ranked in the annual top 10 software chart for all of 2007. Halo 3 sold 4.8 million units through the end of 2007, and it appears that GTA IV will easily outsell it this year.
We estimate that Call of Duty 4, Rainbow Six: Vegas 2, and Rock Band are three of the remaining five titles in the year-to-date top 10. We do not have enough information to confirm their rank nor definitely identify the other two games. We hope that the NPD Group will choose to release a full year-to-date top 10 when it releases figures next month for the first half of 2008.
Since this article was originally published, Frazier informed Next-Gen that "CoD4 has just edged out Wii Play for the fifth spot."