Can alkaline batteries in digital cameras be switched for lithium ion ones?

Yes.
Some digital cameras work much better with Li Ion batteries (Pentax being one). Check to make sure the camera's battery compartment can fit a CRV3.
There are also rechargable CRV3's comming out now. These are sometimes better than NiMH because NiMH batteries often can't supply more than 1.3 volts at a full charge. Regular alkaline have 1.5 volts, but can't supply the mAh that the NiMH batteries do.
The Li Ion supply 3 volts (fits in place of 2 AA batteries). They also can provide a lot of juice for those battery-hungry digital cameras.
I haven't yet, but want to try out a rechargable CRV3. They look like they may be very useful for the Pentax digital SLR's.

honestly, you're better off picking up some rechargeable batteries. but yes, they an be switched if you choose to go that route. rechargeable batteries aren't as expensive as they once used to be.

lithium or rechargable's

NiMH's are the ones you should use