I believe it definitely helps. I am now in the process of obtaining this particular certification. (my application is currently being reviewed)
I personally screened, interviewed and hired 2 GIS technicians. If had an applicant with a GISP certification it would have been a huge benefit to him/her. I base this on the fact that I had to go through a similar application process with a similar IT organization with which a hold a comparable designation.
To touch on some comments with respect to the GISP certification; A person simply cannot obtain the GISP certification just out of college and with no have no experience. Minimum 4 years of full time GIS work is required. More experience will likely be needed if the GIS related tasks are less then 80%. Hence, GISP designation holders are seasoned, experienced professionals with sufficient educations and proven experience. The application process is very detailed and requires original documentation.
GISP is governed by GISCI, a professional organization to which holders of this designation must belong. As a member you subscribe to a code of ethics with which your professional practice must be consistent. (This is also similar to any other professional organization)
While GISP is completely different from the new ESRI certifications the two can happily co-exist. This is because GISP enforces re-certifications process whereby holders of this designation must continue to fulfill the certification requirements through a re-certification process. Ongoing full-time GIS employment, ongoing education as well contribution to the GIS community are essential in order to remain certified. Hence an ESRI certification would count towards GISP re-certification. Employers that knows this can easily check when the applicant become GISP certified and ensure that he or she is on a re-certification path by reviewing recently completed GIS courses, etc.
GISIC is based in the US but it is now trying to promote international involvement. There are many GISP in Canada. In fact there are GISP in 50 US states and 25 other countries. GISP accepts international transcripts and accommodates them by providing formulas for their "educational points" calculations. As far as I know it is the only specifically GIS professional organization in the world.