Speedo -
Note that the following is kind of 'armchair pontificating' for me as far as nature recording, but: AFAIK, the important things for what works for nature sounds are quiet, sensitive mics and quiet, sensistive mic preamps. IOW, capturing low level sounds with high gain and low noise. These happen to be the most demanding things to ask of a both mics and mic preamps. The Sennheisers into a Sound Devices fit this bill perfectly as the all the MKH mics are extremely low noise while having very high sensiitivity (~25mV output!), and the inputs of the SD machines similarly have very low noise but high (72dB) gain. The RF operation of the MKH mics makes them highly resistant to humidity to boot. Probably why the MKHxx series is widely renowned for field recording. The MKHxxxx series is too new to have much track record, but are essentially a smaller format of the same RF mics, so probably are excellent as well. The Rode mentioned by OtheroneK has insanely good output and noise specs too, if you want a hypercardiod pattern.
The Shure VP88 is I think an excellent, bombproof sort of mic, but has extremely low sensitivity, only .5mV, and not excellent self noise specs either. To capture quiet sounds, your preamps would be cranked way up, where even the best preamps start showing their weaknessses (noise). Having to get an outboard preamp to compensate for this (or for low or noisy input gain) to me seems self-defeating in a backpacking type rig. However, the FR2-LE's preamps are well thought of for the price range, and with nice hot mics like the Sennheisers you'd probably do fine. However, like the MKH mics, The Sound Devices are the renowned machines for field work. Highly reliable and capable with excellent sound.
A Sound Devices machine and some MKH mics is my personal dream rig. You could record anything with that setup. If I had the opportunity, it is what I would do hands down.
Peace,
Sanaka