I have had my Nikon D100 for around four years and have shot about 50,000 images.
I think I read once in these forums that digital camera's can wear out.
Is that true,if so when and what gets effected.

Hard to say for sure, I doubt that they will last as long as Film camera...But the moving parts will be sure to have a certain amount of wear and tear. And I am sure we all take far more shots with digital.. But at the rate of 50,000 images in 4 years you will be safe for another 4 or even 8 years.  Canon rate the shutter for 100,000 to 150,000  And quality of build seems to be improving with each new model..

For sure the mechanical parts will be most prone to wear such as AF motors.  Also contacts may also wear out even on flash cards.  Batteries will also only have a certain half life..  I would also think that the elements will degrade the internal electical parts of the camera in time as the seals wear.. Or if you constantly expose them to the heat and cold or high humidity.. 
 
But chances are you will have replaced it with a newer model anywhere near the time it will wear out, and most parts can be replaced it will depend much on what the camera is worth by then.. MY 10D is still going strong and cost me £1200 when new but doubt it will be worth more than £250 now, so if it required repair I would be spending maybe more than it was worth to fix it.. If and when it ever breaks down

I'm with Danny. You'll probably get a new model before you wear out the old one. That D300 looks mighty nice...

Both Danny and Kort have said it....

The very likely first "non consumable" part to go will be the shutter...and as Danny says, they are rated in the 100,000 click range.  So, barring any bad luck, you should be good for another 4 or 5 years.
If you do require any major repair....forget it....you will be able to buy a newer, better model for less than what your D100 cost you.

"Operational" life for these cams is usually greater than "market" life....in other words you will be more likely to upgrade to a fancier version before your old one gives up the ghost.....

And yes, the D300 is very very nice....I think it is time you put one in your hands to see how things have changed in the last 4 years. 
BTW, 4 years is a life time in today's technological advancements.

" And yes, the D300 is very very nice....I think it is time you put one in your hands to see how things have changed in the last 4 years. BTW, 4 years is a life time in today's technological advancements." So true. Not quite the same thing, but I was reading a car article a while ago that was comparing 4 new family sedans. For the most part, their 0-60 times outperformed high performance sports cars from 20 years back, including a Porshe and Ferrari. Technological change is like a snowball rolling down a hill. It keeps getting bigger and bigger and gaining momentum.

Have known shutters to fail at a low attentuation level while others go beyond their limit capacity!

I see that several responders have mentioned shutter failure. 

I can confirm that shutters can and do fail

My medium format film SLR has experienced partial shutter failures in both the
80mm and the 150mm lens. 

The camera and lens were purchased in 1970 or 1971 and repairs were needed
on only two occasions. 

--
Martin