The problem most likely to replace "Y2K" in striking fear into the hearts and minds of North American information system professionals seems to me to be that of the dwindling telephone number space.
Over the last few years, people no doubt will have noticed that new area codes have been proliferating with great rapidity.
This has resulted from the various new uses of telephony technology, notably including:
Families getting a second voice line for the teenagers
Cellular and other mobile telephony
Internet connections (note that each connection consumes two connections...)
It is not uncommon for a "well-connected" individual to have four more telephone numbers, ignoring any work-affiliated numbers.
With this growth of the use of telephony, the growth in the number of telephone numbers is sharply outpacing population growth.
Add to this the growth of telephone-based services, including:
"Normal" business numbers
Toll free numbers (800, 888, and other new ones...)
And the growth in the use of the "phone number space" has been growing at a tremendous rate.
Once illegal digits get taken out, the total number of telephone numbers available in the 999-999-9999 format winds up being something around a billion (which is about a factor of 100 smaller than one might expect).
That "supply" is liable to run out more quickly than anyone yet expects, to the distinct chagrin of:
Would-be producers of new telephony services
Anyone with software written to assume 10 digit phone numbers
Producers of telephone switches
Low grade prediction: The next problem after Y2K gets done will be the "phone number" problem.