The only little workaround that is available, of course, is the chat channels -- those can be created and even moderated in an ad-hoc way (though I believe that they can't be given permanent moderators, which means that if someone leaves the channel, they could lose moderation privledges). But even if a guild alliance is able to keep up a chat channel and keep it moderated, they can't do things like leave a message of the day, or store ingame information on guilds or members as regular guilds can. There is an addon called Guild Alliance that will do lots of that stuff, but there's nothing official.
Should there be an ingame UI for creating alliances? We've been promised more backend guild-based content (like guild battlegrounds and even guild housing), and if Blizzard does choose to heavily update the guild system, there could be a lot of benefit to guilds aligning. Not to mention with the 10/25 main raiding split in the expansion, it's not unexpected that we'll have smaller guilds who join up with each other for the big raids rather than huge guilds with big raiding teams. On the other hand, the practice isn't so widespread at this point -- there may not be enough alliances out there to start officially recognizing them within the game world anyway.