I would like to increase the number of analog inputs on the arduino.
I am hoping to use 25 LDRs.
Also can I use LDRs with digital inputs?
Using functions like "ShiftIn" wouldn't work (to my knowledge), because those are only for digital signals.
That said, to my knowledge, there isn't a way to do it (but I'm sure I'm wrong :P). I'm sure someone will come after me and tell you how to do it.
Also can I use LDRs with digital inputs?
Theoretically, but you'd only get a HIGH or LOW reading, making it pretty much useless. Also, you'd probably only get a LOW reading if it were in COMPLETE darkness.
analog multiplexer
There you have it. Didn't even know those existed :P!
There you have it...
You can buy one as a shield already 
Mowcius
Where can I get one in the UK? Hopefully before Friday!
Maplin sell the 4051 analog multiplexer, if you are near one of their shops, the maplin stock number is QW34M
I can't find any documentation on it.
What are its features?
How do I use it?
see: Arduino Playground - 4051
You can find more info on the cd4051 in google
If someone doesnt beat me to it, Ill be glad to tell you more in depth about how to use it when i get home from work
[edit]did not know there was a playground page for it, so there ya go![/edit]