It is not your PR visa that is expiring, it is your PR card.
PR visa = the visa you get in your passport to go to Canada and land as a PR
PR card = the card you get which proves your PR status and allows you to re-enter Canada after you travel
PR status = the right you have to call yourself a PR
The PR residency requirements are living in Canada for 2 years in any rolling 5 year period. That means that you can not stay outside Canada for more than 3 years at a time without losing your PR status. You can also not renew your PR card when you are outside Canada.
You will have fulfilled your residency requirements for your first 5 years as a PR in Oct 2011 so if you move after that, you can go back to Canada in 2014 and apply to renew your PR card. If you ask them to expedite the application based on your wanting to travel soon, you could possibly get the new card in as little as a month. Then you can leave again and be gone until your total time outside Canada reaches 3 years. Then you must come back.
If you do not come back, once you do not meet the residency requirements any more, you will have lost your PR status. However, since immigration does not have a GPS scanner on every person, they will most likely not realize this unless you were to tell them so if you do stay away longer, as long as you can get back into Canada with your new PR card without being noticed, you can live in Canada for 2 years and meet the residency requirements again.
If you do not renew your PR card when it expires, that does not mean that you have lost your PR status. You can still go back to Canada as long as you have not been away more than 3 years in the last 5 and apply for your PR card renewal but with an expired PR card, you will likely be asked a lot more questions at the border before they let you in.
