In patch 3.3.5, Moonkin PvP is at its most viable. Sadly, it's still not very viable anyways. Druids who want to seriously PvP as Balance have always been met with frustration at the spec's shortcomings. Some Druids have addressed our issues and made long threads about them, some QQed, and others blogged. Either way, the problems of Moonkin PvP are well-known.
However, it never seems as though Blizzard has any intention to bring us up to par. Cataclysm is well out now, and we received some new toys - like everyone else. Now we have a funky silence, funky mushrooms, and an awesome new nuke. But our survivability is still as bad as ever, and our burst damage relies on Shooting Stars procs. Our HoTs are weak and expensive, and the new Eclipse mechanic - though much improved for PvE - is even worse than the old mechanic for PvP.
Sometimes it feels as though Blizzard holds a grudge against Balance Druids. But ultimately, that is a silly notion. Blizzard is a multi-million dollar business, and it isn't in their interest to put down us lowly Moonkins. If they don't satisfy their customers, they might lose money - simple as that. So what is the reason Moonkin PvP has always been unviable?
Why we aren't better
The answer is actually simple. Ghostcrawler has repeated it several times - they do not feel that Moonkin PvP needs to be brought up to par, because Druids already have a strong PvP spec (Restoration). If a Druid wants to PvP, he can just respec or make good use of Dual Talent Specialization. Blizzard cares about class representation in arena - not spec-representation.
This logic has strengths and weaknesses, but it utterly fails with hybrid classes. If a Mage wants to PvP, he can just go Arcane or Frost. Like Balance, Fire is not PvP viable. The difference, however, is that no matter what tree a Mage chooses to specialize in, he will still be doing the same thing - casting damaging spells. A Rogue will always melee DPS, a Hunter will always do ranged damage, etc. Additionally, though Holy is not viable, Priests have another healing tree they can spec - and Discipline is perfectly fine for PvP. Every non-hybrid class in the game can do well in PvP no matter the role they choose within the possibilities of their class (with the exception of tanking specs, which are usually slightly weak).
It's not fair, Blizzard
If a Druid wants to cast damaging spells in PvP, his only choice is Balance. For that reason, I believe it should be viable. Asking us to respec Restoration is not fair - if we wanted to heal people, then that's what we'd do. Shamans have the same problem - though Elemental and Restoration are both arena-viable, Enhancement is not. Our other hybrid friend, however, is doing a little better. Retribution and Holy Paladins are great in PvP - Retribution has a few important flaws (very susceptible to snares), but it remains much more viable than Balance or Enhancement.
If the PvP role you want to play (healing, melee damage, ranged damage) is only offered by one spec within your class, that spec should be made viable.