Are you wanting to continue working on or at least view a PS-CS5 PSD in PS-CS3 and wondering if everything you’ve done will be understood by the older version? I think it depends on what you’ve done, if the operations in the PSD are actually understood by the older version or not. Much of the additions of CS4 and CS5 were usage improvements, not how things are represented in the PSD document, itself, so you might be ok just opening the PS-CS5 PSD back in PS-CS3. Some 3D and animation or video things might be the most different. How complicated of a document is it?
It not all that complicated of a document. I remove all of the bevel and smart object stuff to make it more simple. It just has a very unusual font and a illustrator vector file. I even think i turned the Illustrator file into an eps.
I am sending the font separate so that they can get the font correct. I probably should have rastered the font.
There's really no easy way to know whether you've done things in the new version that won't be understood by the old one, but as has been mentioned there aren't that many things. I can't think of too many offhand, though one that comes to mind is nesting things in groups more deeply.
Let them know it was saved by a newer version and to get back to you for a simpler version if they can't open it.
I still have all the old versions installed here for the times when I want to double check compatibility.
I would include the font, as is, so that it can be installed into the system of the other user. Note: be sure to check on the restrictions of use of the font. That can differ, foundry to foundry, and we do want to keep things perfectly legal.
As others have stated, a generic PSD file should open just fine in either version of the program. Checking Compatibility is always a good idea. *
* Though they came from the same Production Studio, and/or Production Premium (I have both, and cannot recall which program came from which, as they are very similar.), I had some issues with PSD's from Photoshop, that would NOT Place in the same version of InDesign, until I checked the Compatibility option. Not supposed to work that way, but in my case it did. One checkbox, and all was copacetic.