Agree with most here that PR certainly can go down, however in my experience it happens far less frequently. I have an aged domain that I completely disassembled - took down every stitch of content so it just displayed my host's default "This domain coming soon." page. Even after a couple of years without content, the PR was still a 5! Eventually, during one of Google's big updates, it dropped to 0.
I dont agree with this. I have dormant sites that have done the exact opposite of what you described. They havnt had any new backlinks for years, yet the PR on some pages has gone up.
IMO if your PR is going down then that actually might say something about your seo techniques and overall link pop. of a site.
If you've got a site and you've got Google Toolbar watch it. You'll see. If you don't have a site or Google Toolbar choose a site to watch, upload Google Toolbar. You'll see.
I have sites with very few backlinks that got PR 1, 2 and even 3. Other sites, with much more backlinks are still PR0. The former ones, however, are more relevant-content-rich than the latter, I think this is a factor too.
As long as you are actually working for doing SEO on your online business in legal, "white hat" ways I do not think that Google will have any reason to drop your PR, on the contrary. Regards
PR goes up and down all the time depending on how your website does and how much constant backlinks are being added for your website.
PR going down .. may usually happen with sites having higher PR. Have not seen sites with PR upto 4 going down.... I agree partly
PR definitely goes up and down...improve your link popularity and it might go up on the next update if you'll not loose your current IBLs as it'll depends on it...
Pagerank can surely go up or down, depending on many factors including addition or removal of new high quality inbound links. PR, though, is a delayed indication of how your sites have been doing in SERP. It means by the time you get an update of your PR (let's say go up), your sites normally should have already moved up in SERP. Look at this for more details regarding PR: http://www.gordonchoi.com/matt-cutts-explaining-google-pagerank-20061009