MP4 / h.264 patent issues?
IANAL, but as far as I understand it, if you're charging viewers for h.264 / MPEG-4 AVC content you may need to pay license fees. Even though x264 / ffmpeg are Free with a big F, they are just software libraries for encoding video streams into the H.264/MPEG-4 AVC format, which is covered by the MPEG patent. But the threshold for when fees are applicable is fairly high.
According to MPEG-LA, the license holders for H.264 / MPEG-4 AVC, the license fees applicable are:

Where End User pays for AVC Video:
Subscription (not limited by title)

100,000 or fewer subscribers/yr = no royalty;


100,000 to 250,000 subscribers/yr = $25,000;




250,000 to 500,000 subscribers/yr = $50,000;




500,000 to 1M subscribers/yr = $75,000;




1M subscribers/yr = $100,000



Title by Title

12 minutes or less = no royalty;


12 minutes in length = lower of (a) 2% or (b) $0.02 per title



Enterprise cap:

$3.5M/yr 2006 - 07,
$4.25M/yr 2008 - 09,
$5M/yr 2010,
$6.5 million per year 2011 - 2015


They also stated that they:

…will continue not to charge royalties for Internet Video that is free to end users (known as “Internet Broadcast AVC Video”) during the entire life of this License.

So if you're putting it on the internet free you don't need to pay them license fees, ever.
There's other fee schedules for software developers (though Cisco have released free source code and binary implementations of h.264 encoding software and have said they will not pass on any licensing charges for it).