Is all high psa cancer?

No. Not all high psa is cancer. Many times it can be in prostatic hyperplasia, and BPH or benign prostatic hypertrophy. However, many times cancer must be ruled out in these cases. That's why other tests like digital rectal exam and biopsy of the prostate are done. Psa levels >10 ng/ml or positive dre should be tested for cancer with transrectal ultrasound-guided biopsy or trus to rule out cancer.
No, ... The issue and debate on PSA has evolved drastically over the past 10 years although it has been around since 1970. To address the essential information on PSA uncertainty and debate, please review the articles in http://formefirst.com/FreeInfoProstate.html. I do believe you will gain a solid synopsis of PSA.
No. It depends on how one defines "high". A psa of 100 is almost certainly cancer, but a psa of 7 in a 70 year old man for example is of uncertain significance. Many but not all "significant" cancers cause psa elevation but not all psa elevations are cancer. Psa should be interpreted carefully in the context of age, prostate size (exam) and prior psa readings if available.