Why RSA uses {d,n} as private key instead of {e,n}? While studying the RSA algorithm I referred to some books and some sites such as RSA (wikipedia) and all of them chose {d,n} as the secret (private) key and release {e,n} as the public key but as d and e are multiplicative inverse of each other, can't we keep {e,n} as private key and use the other as public key? Is it a standard rule to choose the {d,n} as the private key?
The letters d and e are just a notation -- what matters is that there are two exponents, each undoing what the other does, and making one public does not intrinsically reveal the other. By tradition, the one made public is called e.

However, there is a twist: the exponent that is not made public, i.e. the private key, can be really private only if it is sufficiently big. If the private exponent is very small (e.g. it fits on 20 bits), then it is easy to rebuild it from the public key, if only by trying all possibilities. If the private exponent is not so small but still substantially smaller than the modulus n, more advanced attacks are possible, to the same effect (there is an attack by Coppersmith which works as long as the private exponent length is no more than 29% of the length of n).

Conversely, there is no issue in the public exponent being small. That exponent can be as small as the value 3, although, there again by tradition (and also out of a solid dose of historical confusion), 65537 is often used as public exponent. Small exponents promote faster computations. Since the public exponent can be small, we thus make them very small.

So if you generate your key pair and choose both exponents as big numbers (more or less the same size as n), then you can choose either one to be the public exponent (which will be thereafter called e). However, if you selected one of the exponents to be small, then the small exponent shall be the public one, because otherwise your key would be very weak and prone to cracking.

Some widespread implementations of RSA don't tolerate big public exponents; e.g. the one in Windows CryptoAPI, used by Internet Explorer when doing SSL, requires the public exponent to fit on 32 bits.