I think you'll find it depends on when you unlocked the software. If you logged onto Garmin's site & unlocked before the 19th Jan 03, I think you're shit out of luck.
That said, there seems to be a wide variety of experience on the BB on this subject (with some members getting storming deals - eh Judge?). You should be able to get the V5.0 discs from Garmin foc, try unlocking them & see what happens.....
It was only a storming deal if like me you only had one region unlocked in which case the $150 upgrade will get you both v5 and a full Europe unlock for the same fee - double bubble as the say
I bought my SP III early December 2002. I registered after V5 was launched ( can't remember the date), because of what I read on this forum. Thanks guys. Got the free update. OK.