If you have an org that you need access to without the burden of this restriction (e.g. an org you use to perform demos at various locations and/or from many different computers where you might not have email access), you can authorize all IPs by making 0.0.0.0 the start IP and 255.255.255.255 the end IP in a Network Access record.
Doing this will essentially put that org’s security back to what it was before Salesforce made their enhancements. I wouldn’t recommend making this change unless you actually need it, but it can come in handy even if for a short-term fix.
This is the approach Salesforce.com has taken for Test Drive orgs on AppExchange listings so that Test Drive users are not restricted access to a demo. All of my Test Drives had a record with this range pre-populated by Salesforce when they made their security changes.
Note: I do not recommend anyone do this for their corporate Salesforce.com org as this would negate all the good work Salesforce has done to button things up.