I was looking through the labs on Gmail and I found a lab called "SmartLabels" which I thought might be of interest to webOS & Gmail users. It was released a couple of months ago and the main purpose is to use Gmail's algorithms to filter out bulk mail like it does for junk mail and there is some ability to train it I believe (it also adds labels for notifications and forums). I find it really useful for my Pre because the only emails that make it to my Inbox and therefore my phone are personal emails and things that are "important". I like to read my "bulk mail" (newsletters, mass mailings, etc) when I'm at my computer, not on my phone.

It works very much like any other filter and you can still keep your other filters (you do have the option to bypass the SmartLabel filter on your other filters). I never found the Priority Inbox useful because it only works well on the web interface which I never use (I only use Thunderbird and my Pre). I just enabled it today so I'm waiting to see how well it works, but it did pretty well marking all of my previous emails.

To enable it, go to Mail Settings->Labs and search for "SmartLabels".

From the Gmail Blog:
New in Gmail Labs: Smart Labels - Official Gmail Blog