Play In Any Key in One Place on a Guitar

So you know your instrument? What if someone shouts a blues in Eb? Or if you get a gig with a singer who insists on doing "Sunny" in Ab minor?

Steps

Get your guitar and your metronome and switch your head into practice mode.
Start in 2nd position (1st finger 2nd fret) - most people know their notes better there - Each finger covers a fret, and a one fret stretch in either direction is allowed. All other frets are out of bounds.
Set the metronome at a comfortable tempo.
Play scale of C major and extend it past the root note in both directions to encompass all notes in range of given position (you will get as low as F 1st fret low E string)
When you hit low F, come back up F major (1b) without losing time if you can help it.
The next scale will be Bb (we're moving round the cycle of fifths - if you don't know loads of info on the web on this). Come up from your low F and change key when you hit the Bb.
Continue in this way around the cycle - C,F,Bb,Eb,Ab,Db,Gb,B,E,A,D,G - in time.
Try it chromatically - C,Db,D,etc.
You've just played in every key in one position. Have a cup of tea.
Repeat daily.
Learn in different positions and for different scales (melodic minor, any of the modes etc.). Also try variations (scales in groups 123,234,345 etc. or in 3rds or 4ths etc.)

Play slowly and in time - Speed will come naturally when your fingers are ready.
Practice little and often. Half an hour daily is much better then all of Saturday afternoon
This is as much a physical exercise as a mental one. You will be forced by the constraints of position to play every possible static fingering of the scale you're practicing. Pay attention to this, watch your fingering and don't hurt yourself! (possible)
If you can't, learn to read music. Bit of a drag if you can already play, but really helps understanding of this kind of thing.
If you're struggling identifying notes there are loads of nice things like theory information and fretboard maps on the web. Do some research.
Chords can be practiced in a similar fashion.

This is just an exercise - put it into practice by playing 3 tunes you know every day in a different key. Try changing key for the last chorus of a tune and staying in position. Otherwise it remains just that - an exercise.
Don't overdo it - learn to listen to your body. RSI, tendinitis, carpal tunnel are real things that musicians get. Alexander technique is good if you have posture problems.
There are a lot of scales (four parent scales have 7 modes each, two pentatonics, whole tone, chromatic, octatonic each in 12 different keys and how many possible positions...) Don't panic! This is not a tool for learning the neck parrot fashion, but a way of organizing the fingerboard.