First, I just posted a new NilRTOS Google Code Archive - Long-term storage for Google Code Project Hosting.. Look at the new nilSdLogger.ino example, it uses the FIFO template and NilTimer1.
The problem is CPU power to format numbers for these examples.
My new SdFat printField() is about three times faster than Arduino Print but is still the bottleneck when you have lots of RAM.
FIFO record count: 833
Minimum free record count: 0
Maximum SD write latency: 48860 usec
Unused Stack: 51 5341
The 833 FIFO records at 1024 usec provides about 850 milliseconds of buffering. The max latency was under 50 milliseconds so text formatting is the problem.