After discussion with irssi developers, both projects are going to merge in the next weeks.
Most of WeeChat features will not be available in first merged versions, but they will be in a few months.
This merge will give missing features to WeeChat and give new from irssi.
WeeChat developers will not take part into final client (which should be renamed to "weerssi").
Update: it is of course an april joke :-)
m'enfou j'utilise irssi !!! c'est bien mieux que weechat !!
it's a joke ! weechat c'est bien mangez en !