In response to the work being done by cschur:
First, thanks for undertaking this! I'm building out two new robots: one's a line follower, another is a 4WD chassis initially intended for mini-sumo, but it's also being designed as a general purpose explorer. I was planning to use IR sensors, so I eagerly await your findings.
Second, have you looked at the Sharp GP2D12 and related family of sensors? They use triangulation rather than modulated reflectivity, and seem to be much less sensitive to the reflectivity of the target surface. At one point I was trying to "stealthify" a mini-sumo robot, and found it could accurately gauge distance even when the target had extremely low reflectivity. It was uncanny.
Finally, I seriously look forward to your IR photographs. I've done IR film photography since about 1996, and switched to digital IR in 1999. These days I work at a place that does a lot of IR optics. As part of that I run spectra from 190nm out to 3200nm of any material we put close to an optical beam. As you say, the results can be surprising.
Black electrical tape is black in the IR. Colored electrical tape is not. Antistatic foam, as you pointed out, is quite dark and makes good optical baffle material. Some flock papers are extremely bright in the IR. Others (typically PVC-based ones) are extremely black. Black Delrin is black in the IR. Not all black plastics are. Most black anodization dyes are not black in the IR. Black cotton cloths tend to be quite black in the IR. Black synthetics like satin tend to be extremely reflective. Krylon Ultraflat Black paint has almost the same spectral response as higher priced IR paints (pretty sure they both use carbon as a pigment.)
I'd be curious to see what your findings reveal.