I have heard stories of people being kicked out of guilds for being caught playing PvP on one of their alts that happens to be in a different alliance.
I even asked some people I was in a group chat about this and they seemed to feel that this is perfectly appropriate.
I am not sure what to think. I have Alts in all three alliances and members of guilds spread across alliances too. This isn't some kind of tactic to interfere with the flow of campaigns but simply to keep my options open. Given (until one Tamriel) it is only possible to play with friends with characters that are in the same alliance I like knowing in principle I should be able to play with anyone. I don't play enough PvP to have had much issue anyway but hoped to get some feedback.
It was implied by some that any time spent in PvP should be devoted to your guild. I even heard remarks of creating separate accounts if you want to play a different alliance.
The practicality of dedicating yourself exclusively to one alliance means that any alts you have in a different alliance can never be trained up with PvP abilities. This is not the end of the world.
I may personally think it is just a game and I should be able to play with friends in whatever faction I want and not take it so seriously. At the same time if it is really so badly frowned upon or may cause too much fall out I can live with limiting myself to just PvP in one alliance.
My Characters
Xargothius: Breton - JOAT / Magsorc | Orchid the Fair: Orismer - Crafter / Heavy Tank | Voneri Vox: Dunmer - Magplar Healer | Rexorgiana: Imperial - Temptress / Magblade
Phirkius: Altmer - MagSorc | Rexorigus: Imperial - Med Tank / StamKnight | Sven Svedishchef: Nord - Provisioner / Stamplar | Finds-All-Fungi: Argonian - Alchemist/ Stamblade
Emerald-Wild-Guard: Bosmer - Wyrd / StamSorc | Nates Datum Festi: Imperial - Light Tank / StamKnight | Magnolia Desert-Blossom: Redguard - 2 Handed Stamplar | Shadow-Softpaw: Khajiit - StamBlade
Sorcerer | Dragon Knight | Templar | Night Blade