Unfortunately, a PLB is not an ELT-S.
The definition of and specifications for a Survival ELT (ELT-S) are set out in RTCA Document DO-204 and EUROCAE Document ED-62. These standards spell out various ELT-S requirements that are not met by PLBs, including crash survivability and operating ability in environmental extremes, 121.5MHz homer output power and transmission of the aircraft's ICAO 24-bit address.
Products that are approved as ELT-S beacons in compliance with those specifications include the Kannad 406AS, the Artex SLB-406 and the ELTA ADT406S. None of those examples are GPS-enabled; ergo, it will take some 30 to 60 minutes before the LEOSAR element of the COSPAS-SARSAT system could accurately determine your position using doppler shift.
On the other hand, whilst the GPS-enabled PLBs most of us are already using or considering purchasing are NOT approved ELT-S beacons in compliance with those specifications, they will transmit an extremely accurate position very soon after activation.
An ICAO/IMO Working Group in June 2006 concluded "there was a strong case for the allowance of PLBs as a means of compliance with requirements for non-automatic ELT carriage by GA aircraft." Regrettably, to date their findings have not been adopted by ICAO, and the Dutch and the British (unlike the French) have adopted the ICAO stance verbatim. Fortunately, at the very least until 1 May 2009, the UK CAA have granted a non-public transport exemption from the ANO requirement to carry ELTs.