Sorry to bring up an old thread, but Netscaler has it now. Hopefully this means we will see it soon.
it is tracked as ID389414 and under development.
ID389414 - RFE: LTM to support Multipath TCP (draft-ietf-mptcp-multiaddressed-09)
Is it possible to follow the development somehow?
f5 BIG-IP LTM supports MPTCP (Multipath TCP) from version 11.5.0 forward, starting in May 2014. See http://support.f5.com/kb/en-us/products/big-ip_ltm/releasenotes/product/relnote-ltm-11-5-0.html#rn_new and the LTM documentation for configuration details.
Have a read