BGP prefix hijacking has been perceived as a growing threat over the last year or so. A prefix hijacking occurs when an unauthorized party advertises one or more prefixes belonging to another party. By advertising the prefixes with more favorable characteristics (such as longer matches or a shorter AS path), traffic across the Internet can be redirected for malicious purposes, effectively creating a man-in-the-middle attack with global reach.
The highest-profile example of such a scenario occurred last February when Pakistan Telecom, in an effort to block access to YouTube.com from the entire country, accidentally advertised to the Internet a "black hole" route for YouTube's public address space. All (or most) traffic destined for YouTube.com was being routed to Pakistan, where it was subsequently discarded. Needless to say, some people got pissed.
Incidents such as this have prompted much discussion, but currently this vulnerability still persists throughout the Internet. In the absence of a solution, more attention is being paid to monitoring BGP advertisements. Organizations and individuals have been writing code to leverage the insight of public looking glasses and automate the monitoring of address prefixes.
One such service recently debuted at BGPmon.net, managed by Andree Toonk. BGPmon provides free monitoring of select BGP prefixes and automated E-mail notification when suspicious changes are observed. For example, assume your AS 123 normally advertises 126.96.36.199/16 to the Internet. If AS 456 begins advertising two routes, 188.8.131.52/17 and 184.108.40.206/17, traffic will follow these more-specific routes to AS 456. BGPmon would detect this as a possible MITM attack and alert the administrators of AS 123.
BGPmon also provides some interesting weathermaps and general statistics of the global routing table. Even if you don't administer an autonomous system, the ongoing growth and reshaping of the world's routing table can be fascinating to observe.