Wormholes are everywhere, throughout the so-called cosmic foam. There are untold gazillions between your eyes and the computer screen. Unfortunately they are far to small to see, to have any physical effect on the universe or to be traversable. From here
http://library.thinkquest.org/12523/article3.html
The secret in the creation of such a wormhole lies in quantum foam, a property of space in tiny scales. The amount detail that we can recognize depends on the amount of magnification. The magnification which is necessary to analyse the quantum foam cannot be attained by any microscope. It's simply impossible to make such small structures visible, because you would - after having zoomed to the size of a nucleus (10^-13 cm) - have to magnify another 20 times by the factor of 10 until you reach the quantum foam level.
To make a wormhole traversable, you would have to expand it using 'negative energy'. Meteorwayne has it slightly wrong; the energy required to inflate a wormhole isn't 'greater than all the universe'; it is much more difficult than that. To inflate a wormhole you need only about as much energy as contained in a moderate sized gas giant- but it needs to be 'negative in quality', an absence of energy on a massive scale.
Now it is currently possible to create tiny amounts of 'negative' energy in the lab, using the Casimir effect; this peculiar effect produces a tiny bit of space with less energy than the rest of the universe. To create enough negative energy to hold open a wormhole, wide enough to allow a human to pass through- well that would be very difficult indeed.
On the other hand a number of physicists have tried to get round this problem by investigating the possibility of traversable wormholes using 'arbitrarily small' amounts of negative energy.
see this paper for instance
http://arxiv.org/abs/gr-qc/0301003
Can it be done? I don't know.
Chris Fewster, from my home town of York, and Thomas Roman have examined some of these models and found that they would be very difficult to traverse and would be severely constrained. Whether this constraint is fatal to the concept remains to be seen.
http://arxiv.org/abs/gr-qc/0507013