I think the effectiveness of turtling depends on the game and the map. Some games (C&C 3, for example) have powerful enough defenses to make turtling viable, while others it is very difficult to manage (Homeworld is a good example). Some maps are ideal for turtling because of terrain which can make concentrated defenses murderous, (for instance, if the only way into your base is one very narrow pass). Some larger maps it's no good because resources are scattered far and wide and it's better to have mobile units that can defend a larger area rather than concentrating all your heavy defense in a few small baskets. Varying your strategy depending on the game and the map is very important to being successul.
As for delivering the kill shot from a turtle strategy, I've found that usually there are three options (generally speaking):
1. The Steamroller: Classic, but there's a reason for that. Consolidate your logistics, build up your forces, then go for broke. Furthermore, unless you completely deplete your resources, you're still left in a strong defensive position to rebuild and try again (unlike a rush, which leaves you wide open). The downside: This requires significant amounts of resources, and you'll be vulnerable while you establish your supply line.
2. The Guerrilla: This strategy usually focuses on chopping the enemy's resource line and building up a small but powerful force to exploit your advantage once the enemy is vulnerable. Isolating powerful units from their escorts and annihilating them is also effective, but the cornerstone here is resource denial. The more time and money your opponent spends trying to keep his supply line intact is less time and money going into offensive units. Mobile but hard hitting units are most effective at this (bombers, attack helicopters, mobile artillery, stealth units, etc.). The downside: This requires a lot of careful micromanagement and multi-tasking, and if your mobile units run into enemy heavy forces, you're going to have trouble.
3. Pure Turtle: How does this work you ask? Slowly advance into enemy territory, destroying opposition as you go, and then fortifying each position you take with base defenses and other fortifications. The goal: Eventually creep closer and closer to the enemy base, and try to lock them in with your base defenses. Then, send in your main forces and superweapons to deliver the killshot. The downside: This requires TREMENDOUS amounts of resources, time, patience, and skill. Furthermore, I've never really seen it work against anything but an easy AI. Although I've never really been bold enough to try it against a human player.
Anyways, sorry for the text wall, but those are my thoughts on my preferred RTS strategy