After 3 weeks running Ubuntu 9.04 Jaunty Jackalope on hp 540, here's the point by point review.

Graphics :
The built in Intel GME965/GLE960 Integrated Graphics Controller recognized and visual effect (compiz) can run smoothly. Tried it with more graphical intensive Kubuntu and it works.
Video playback runs good too.

Sounds :
Sounds work, it may sounds too harsh if the volume is set to maximal. I guess it is because the driver, not the hardware itself. Listening music via headphones is enjoyable.
I tried voice chat and it works, mind you there's no built in microphone, I use a separate headset.

Wifi :
I haven't try to connect via wifi, but from my house it can detect and recognize Wireless Networks from not so near Internet Cafes (so the reception must be good), either unprotected or protected.
Here's some networks automatically detected around my house, but since I don't know the their setting I couldn't connect, that and being a good citizen.
Maybe some other time when I am in free hotspot area.
 

Touchpad and scrollpad:
Works.

Function keys (fn+) :
Works.

Keyboard :
The alt key sometimes lock itself, pressing it several times will unlock it. (edit : it appears it is not Ubuntu related, but the hardware)

External Modem:
Although it's not part of hp 540, but since I use it almost every time it is on I include it here. I've tried Motorola L7, Motorola V3XX and Huawei E220 as a modem and they work.

Bluetooth :
Exchanging files with bluetooth-enabled device (a Nokia 6230) is working. Although it needed some tweak to be able to use it as bluetooth modem ( I haven't try it yet)

Suspend and hibernate :
Suspending works, but hibernating is not.
 
Problems :
Some problems I experienced so far :

• random freezes with only mouse can moves around, but clicks and keyboard are not responsive. Mind you I'm no expert so these are my guesses of what may causing the problem:
  • Intel graphics driver (most likely),
  • something related with usb driver, this just happened : it hanged when I plugged an external hard disk in. But not always.
    
  • faulty(?) memory module, two weeks before I added 1 GB RAM
    
• Alt key lock it self randomly, pressing the alt key several time will solve this problem.
  

That's it for now, I'll update this post when something new come up. What do you want to know?

update :

I think the freezes problem is solved now. Already tested for two days on my usual usage without any problem. But I'm not sure, I'll wait another days to make sure.
Here's what I did:

• Use Intel Graphics driver from ubuntu-x-swat repository
• enable uxa, xaa, exa by editing /etc/x11/xorg.conf
  ...
  Section "Device"
      Identifier    "Configured Video Device"
      Option "AccelMethod"    "UXA"
      Option "AccelMethod"    "XAA"
      Option "AccelMethod"    "EXA"
  EndSection
  ...
  

Remember it's only been tested for two days so don't let your hope too high and I'm not responsible for what would happens to your Ubuntu. I deliberately wrote the method  in a way that would only understood by those who know what their doing. I'll write in more easy to understood post when I'm sure that this method works.
One thing, it breaks suspend mode.

Thanks to John, I already eliminated memory as a possible problem source.

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