[Arkady]

Reciprocity Failure is when the metered exposure fails to deliver the expected result because of the film not behaving according to the rule of 'reciprocity'. It is different for every type of film and tables are available from the manufacturers to tell you how much extra exposure to apply in various conditions, typically slower than 1 second and faster than 1/100,000 sec. Most of us only experienced this with slow shutter speeds as we didn't have access to ultra-high shutter speeds in the Good ol' Days.
With digital there is a different problem, that of increased digital noise with long exposures. This is (roughly speaking) caused by the pixels recording too much information at each pixel site and rendering a false colour value as a result, which manifests itself as 'noise'. Modern Pro cameras have more sophisticated software to take account of this: some manufacturers even render a 'black' pixel for each colour value and combine this during interpolation to lessen 'noise' on long exposures - the reasoning being that black looks better than red, green or blue against a dark background.
When we tested the D1X in 2002, we recorded images at up to 32 seconds with no significant noise at 125ISO, which gave better results than rating the chip at a higher speed, say 800ISO.
Since the firmware update two years ago, it's even better and cameras bought in the last year should be even better.
Cheaper cameras - less than £800, say - may be less suited to long exposures - do a test before you buy, if in doubt..