In reply to Toerag:
> Interestingly pegs should be hammered in vertically and have a notch for whatever's tied to them.
I think it's useful to look at the geometry of the guyline. Since the guyline is attached to the tent above the ground, if you lift the guyline up at the peg, it will move away from the tent (in a circular arc).
If you place a peg vertically in the ground, tension on the guyline will lift the peg out of the ground (resolve the forces on the guyline into horizontal and vertical components). The guyline can become slacker as the peg lifts, so this is how it will move if the line is tensioned by wind on the tent (the system moves to a lower energy configuration).
To stop the peg lifting, it should be inserted so that the angle between the guyline and the lower half of the peg is just under 90 degrees at the point of insertion, i.e. leaning out a little. This will depend on the length of the guyine and the relative heights of attachment and pegging points. For the peg to be pulled out in this configuration, it has to stretch the guyline, thus moving to a higher energy configuration, which isn't what the system wants to do. The "shepherd's crook" on the end of the peg is to prevent a hooked peg turning when inserted like this.
Bolts are best fitted horizontally because the load is primarily vertical (and therefore bolt loading occurs perpendicular to its placement). I suspect that even bolts may benefit from being inserted slightly inclined, with exposed tip slightly higher than inserted end (if only to ensure that they aren't angled down).