So, I have looked around on these forums and some other sites such as wowwiki and I can't find an answer to "does dual-wielding increase miss chance to *both* offhand and main hand or just offhand?" If anyone knows the answer or can direct me to a relevant post that would be much appreciated.

Yes, both weapon's white attacks are affected.

So long as it's brought up, it's 24% base for an even level mob with dual wielding but what's the miss rate for +3s?

Best data points towards 25.5% for +3s and skull level mobs. For that data, look in the Rogue DPS Spreadsheet thread.... oy, around page 40ish?

Thank you very much, now I have a very good defence when I'm arguing with a fellow rogue about why hit rating is important.

Note that the 24% DW penalty applies to white damage (auto-attack) only. Yellow damage (specials) still use the base 5% (8.6%) miss rate. That's probably what your fellow rogue is referring to. However, hit rating is still extremely important simply because your white damage is the majority of your total damage output.

Regardless we're still looking at a 25-26% or so cap for raid hit, correct?

Yeah I know it's only white damage, though my white damage makes up atleast 50% of my damage, so I'm rather concerned about improving it!

Not sure exactly what he was thinking, he said he hit 140 hit rating and now wants to build up AP. We reset damage meters for the last boss of DM (he was on adds) and I beat him by at least a couple of percent... He also had two 71 dps weapons to my 66 MH and 58 OH... I think that proved my point somewhat.

One a side note: could anyone post some of the conclusions on the actual hit cap that may have been reached in any of the threads? I was reading some, but they seem to get sidetracked quite often...

+hit is more important for a combat rogue, primarily because of Combat Potency, so it's a cheap stat (especially for the first 9% or so when it's more or less equal to +crit) for a lot of very nice return. Also, more hits proc "on hit" procs more often!