Common enough... I have just had the problem occur on a 17" Powerbook that is only 5 months old. Looking at various forums and knowledge base articles as well as complaints, I could only find dire warnings and negative comment.
Working on the principle that simple things are simple and not wanting to be without my machine for any length of time, or ship my valuable computer to Apple, I examined the latch. Looking down into the two holes for the latch on the palm rest, I could see that there appeared to be a misalignment of the latching mechanism. The left side was further back than the right side and neither side was flush with the grey plastic trim.The latch button also sat at an angle that corresponded to the findings outlined.
The left side of the button at the front of the lower case was depressed more than the right.
The various articles and postings I found had all talked of bending the hinge hooks in the upper part of the case ( the screen display) towards the screen (from the front edge). It seemed a really drastic step to take and in my opinion it is both a heavy handed and an unnecessary approach to what I now see as a very simple problem. I would suggest that you do not do it until you have at least tried the following method of releasing a latch that does not seem to keep the case closed.
1. Press the latch button inwards and hold it in that position
2. Blow some air into both holes of the palm rest
3. Release the latch button
4. Close case.
The reason I think this will work in many cases is this...
The possibility of small particles of dust or dirt entering the holes on the palm rest must exist because the holes are present. I think that repeated pressing of the latch button may cause dust particles (which have already entered the holes in the palm rest and are sitting on top of the latch mechanism) to displace and fall between the latch mechanism and the plastic trim. This has the effect of stopping the latch from closing correctly because it is being blocked by dirt. Using a little compressed air (I just blew sharply into both holes) moves the obstruction and the case opening and closing mechanism acts normally.
It may be a design issue but I don't mind if I have to do that once every few months. I still think that the Powerbook is the finest laptop I have ever used.