If your opponent turtles, expand and pump a bigger army and get him when he tries to attack you, don't try to bust into a turtling base. harass maybe, but don't try to end the game.
The major disadvantage to someone that turtles is their economy, as long as you can expand early and get large amounts of resources you can overwhelm him. Make sure you don't let them expand though, they may try to pull extra resources from a mineral patch on a far end of the map.
In my experience, enough Battle Cruisers can crack any turtled base, Expand a lot and mass a lot of battle cruisers. Micro them efficiently so they Yamato the missile turrets to minimize damage they may take. Turtling limits his force and his economy, he's basically inviting you to overwhelm him. Yamato is turrets and fly on in!
A good way to harrass a turtler early on, is with tanks, maybe
with some air cover, like vikings, that can switch to ground (or BCs
if you have them), and as said, expand, expand, expand, make an
awesome economy, 2-3 bases full on minerals, 3+ full on gas, 5+
starports pumping BCs, all upgrades going, etc.
I recently won a few matches against some pretty skilled turtlers
by expanding early, and most importantly: stifling their attempts to
expand. The game was longer than it should have been, but there is
only so much a Turtler can do when their plans to get additional
resources are thwarted.
It is really risky as a turtler only has so much room to scan in
order to find you, but nuking is quite a good strategy to use for
mitigating the turtle economy.
Hope this Guide helps beat those nasty campers!
General Starcraft 2 Guides