Brian Crecente at Rocky Mountain News (and of course, Kotaku) does a quick, but interesting interview with Bizarre Creations, the creators of Geometry Wars (not to mention PGR3...).  If you're curious about what it takes to build a game like GeoWars, you should check out "Bizarre Creations' bizarre creation..."

How long did it take and how many people worked on creating Geometry Wars: Retro Evolved?

It was mainly Stephen (Cakebread) as all the graphics are coded, although obviously we had some music written by Audio Antics and a couple of artists did the Live Arcade and achievements artwork. It wasn't an exact 'start-to-finish' project, as Stephen added in bits when he had time, but all-in-all it was probably about 3 months.

How does that compare to PGR 3?

Oh, miles apart! We had a total of 80 people working on PGR3 at one point, along with some outsourced graphics as well. The game took 2 years from start to finish, and probably averaged a team size of 40 - so it's 3 versus nearly 1,000 man months!!!