I think I'm done with making long posts, as I've already covered just about all of the flaws that I think make it testable. I don't know what the standard procedure is, whether somebody would make a tournament so we could get some more evidence or whether there's enough here for it to be a suspect. Either way, I would be repeating myself until something did happen.
It does get Roost, backed up by 90 base speed and 106/90/154 defences and Fire/Flying is a damn good defence type if not for SR - Infernape, Bronzong, Scizor, Shaymin, Heatran are all common pokemon that can't do jack to Ho-oh whilst it Roosts in their faces.
Oddly enough, not as much as you'd think. If Ho-oh switches into Stealth Rock, it is actually OHKOd by Scizor's +2 LO Quick Attack. I've been curious myself about how well it can counter Skymin (because if we let it into OU some people might want Skymin tested again afterwards).
If my calcs are correct, Timid LO Skymin hitting 252 HP Ho-oh on the switch with Seed Flare does 10% damage.
After the Sp. Def drop Skymin does
207-244 damage with LO Air Slash. If Ho-oh flinches, it's an easy KO. I expect that even without Life Orb it wouldn't be a good counter for it.
So now we can establish that
Ho-oh is not a Skymin counter and Ho-oh
can only counter Scizor if there is no Stealth Rock.
You can switch Ho-oh into Infernape, but if it happens to use Swords Dance as you come in you could be at risk of LO T-punch OHKOing you, even with 252 HP EVs (well, anything less than that and the OHKO is guaranteed). It can run Stone Edge too.
Basically, Ho-oh is not the answer to all of Platinum's new big threats, as Skymin, Kingdra and often not even Scizor are countered by it before it gets beaten by them.
(and yes, this does mean I'm no longer concerned about the possibility of Skymin being banned before Ho-oh gets in)
@Sanjay: Look, you seem to have forgotten a lot. First of all,
why do people keep saying Stealth Rock is the only weakness I can come up with? It's quite frustrating. I've come up with counters (many of which aren't spoiled by a burn, might I add), revenge killers, and calculations like our one above here where we see that Ho-oh can actually be beaten by Skymin and its neutral Air Slash if it comes in afterwards.
Your LO set with Roost is not bulky enough to take hits from a burned Rhyperior, as it needs to have 252 HP EVs to stall that out. Your set also lacks a good way to beat Tyranitar, who can win one on one thanks to your more offensively-minded set. And you don't need Rhyperior, either; I've made a post full of things that can handle LO mixed sets.
Rhyperior doesn't need its moves to miss to be a counter. Let's look at all the sets on its page... we have Rock Polishers, Choice Banders, Resttalkers and Swords Dancers. Note how the only one of its sets Ho-oh has a slight chance of beating is the standard one.