## Ruby, <s>96</s> 75 characters

    require"date"
    d=Date.parse(gets||"thu")+1
    d+=1 while d.wday*d.day!=65
    $><<d

Takes the date from stdin. To not specify a date press ctrl-d.

Thanks very much for Ventero's help.

Ungolfed:

    require "date"
    # Date.parse("thu") will return this week's thursday
    date = Date.parse(gets || "thu")+1
    date += 1 while d.wday * d.day != 5 * 13
    $stdout << date

Sample IO:

    $ ruby fr13th.rb
    2013-05-09
    2013-09-13
    $ ruby fr13th.rb
    2007-06-29
    2007-07-13
    $ ruby fr13th.rb
    2007-07-13
    2008-06-13
    $ ruby fr13th.rb
    2011-05-13