To elaborate even more, let's take all gear that's T5 and up w/ +spell hit on it and add it up (we'll exclude gems and gem set bonuses) and only include pieces people would actually use for tanking:
Plate:
http://www.wowhead.com/?items=4.4&filter=qu=4;minle=128;cr=48;crs=1;crv=0
Girdle of the Protector: 18
Sabatons of the Righteous Defender: 18
Weapons:
Tempest of Chaos: 17
Amani Punisher: 20
Hammer of Judgement: 22
That's about it. You'd have to consider pure spell damage items after that. And the trinkets w/ spell hit on them that are T5 and up are not very useful threat-wise.
So, we're talking 35 - 40 spell hit. This is far shy of 101 to hit a 9% base miss.
Looking into the future, WotLK and level 80 will introduce higher rates of combat ratings. Meaning, it will still take a 17% base chance to hit a raid boss with spells, but the rating will be at least 19.91 rating for each 1% to hit (based on the current ratio). That's 160 hit rating to meet the base miss of melee. We'll never have enough of an itemization budget to warrant RD switching back to +spell unless we are impacted by some serious class changes (which we don't really need).
Is it just me, or are we the only class that settles w/ skating by on crumbs of hit rating (spell and melee)? Why even bother itemize it at all? The weapons we use can be used by other casters and therefore may warrant spell hit. But why the hell do we have x2 plate pieces w/ spell hit smack in the middle of gear progression?
Anywho, that's my rant.