I've seen this topic get quite a kicking around on these formus, and I've been struggling with this one since February. I would see media pending anytime I imported HDV footage. I would see it when I added certain effects like 'Solid Composite" and then prerendered the footage. I worked with Adobe and they were pretty much useless. I figured out some fixes on my own, like I noticed that I could capture and work with HDV but couldn't import it. That was related somehow to those XML files the capture created. I discovered that nesting footage would help. I tried avoiding HDV altogether converting my HDV to H.264, but those files can get very large and the conversion can take a LONG time.
Yesterday I found out what may be causing this. First of all, if you don't use Windows this won't apply to you. As an After Effects user, I have always turned on the /3GB switch in my boot.ini so that AE can get more than 2GB RAM. I've been doing this for years and this has never been a problem. After reading somewhere that it can have a negative affect on a system with a video card that has 1GB RAM, I decided to experiment. To my amazement, it solved all those media pending problems that I had. So far so good, I can import HDV, I can prerender without nesting, I can apply effects to my hearts content, and there is no sign of the problem reoccuring. It has only been a day, but given the fact that these problems were 100% reproducable on multiple systems with different hardware and different CS4 revisions and now they are gone, I'm pretty optimistic. Hope this will help many of you. I am curious if it does so let me know.
BTW, if you didn't glean this from the way I worded my post, the experiment was to remove the /3GB switch from the boot.ini file and reboot.
Removing the /3GB switch while troubleshooting has long been one of the Quick Tips in this troubleshooting page.
There are other tips than seem like no brainers, but sometimes people change a seemingly inocuous setting for some reason or other, then they forget they did it and it starts causing a problem down the line. The troubleshooting page is designed to have people check everything to ensure settings are set correctly.
Glad you found the cause of your problems and thanks for posting your success.