Apart from DES which is known to be weak for a number of reasons most other crypto systems in use today are very strong,I don't know of any back door to the systems used in pgp,which are triple DES,cast and idea.
The way government agencies get around the problem is to try to limit the allowable key size,arguing that 128 bit crypto is strong enough to stop a private individual but not the NSA for example.Gven that as a rule of thumb every additional bit increase in key size means about ten times as much work in a brute force simple key search,or maybe five times increase if you use something like differencial cryptanalysis,then pgp with a maximum key size of 4096 bits is pretty much unbreakable within the lifetime of the sun,using current technology.Its thought but never been proved tha all the crypto systems used in pgp are what is known as NP complete,which basically means there is probably no short cut.