Eneloops will provide no performance advantages over NiMH, whereas Li-Ion batteries do, because they have a higher output voltage of 1.5v per AA cell (and 3.0v per CRV3) as opposed to 1.2v per AA cell of the Eneloops and NiMH. The Pentax range of D series and the K100D/110D are designed around a nominal 6.0v voltage supply, not 4.8v.
What does this lower voltage affect? Mainly auto-focus speed (almost halves it) and sometimes gives false battery depletion indication, especially if you shoot rapidly or you are operating in low temperatures. It can also cause the AF system to hunt.
Many rechargeable CRV3's (RCRV3) have an even higher output voltage of up to nearly 4v (2 will give 8v) and can cause the camera to shut down, or maybe could even damage the electronics. That's why Pentax advise against the use of RCRV3's. The Delkin ones, and the type that Jonas B uses, are voltage regulated at 3.2v (the same as a normal CRV3) and cause no problems.
If you don't mind slow AF or use manual focuss, then NiMH's are okay. If you like snappy AF (for Pentax!) use either AA or CRV3 disposable Lithiums. If you going to shoot over 25,000 shots with your DSLR then the Delkins (or similar) are a consideration financially.
If you are considering the new K10D, it's of no concern as it doesn't use AA's or CRV3's.
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Richard Day - 'Carpe Diem!'
Gloucester UK
