While different users have reported varying symptoms and differing circumstances, it essentially boils down to this: if you download a group of files, and then attempt to open the entire group of files at once (rather than opening each file individually), chances are either some or all of the selected files will not open.
My initial response to this was that it could possibly be one of those "it's not a bug, it's a feature" ordeals. We have already seen some indications that Apple is working on enhanced security and virus protection in Snow Leopard, so it stands to reason that the operating system may be simply trying to prevent what it thinks is malicious activity by imposing restrictions on what you can and can't do with a newly downloaded file.
Of course, that's just my theory, but I'm apparently not the only one who has explored that possibility -- a reader on the Mac OS X Hints forum discovered some hard evidence supporting that theory. In OS X, newly downloaded files are flagged with a "quarantine" attribute, which remains until the file is opened. If this attribute is set when the file is launched, and the file is an application, you would receive the typical "This application was downloaded from the Internet" confirmation before the application is allowed to continue launching. This is nothing new in Snow Leopard, but it would seem that the additional protections and scanning that were added to Snow Leopard most likely prevent opening multiple files so that each file can be individually checked before it is launched.
Whether or not this is a serious bug or a minor inconvenience depends entirely on your workflow. Personally, I find that I don't often open multiple files from Finder, as most of the applications I use have the ability to select multiple files from their Open dialog. I can easily see where this might pose significant problems, however. For example, if you do a lot of work with graphics, you might download a several photos from a client, highlight them and open them with a custom tool that automatically resizes and processes for you. With this bug in play, that task would become much more difficult if you had to open each file individually.
So what about you? Is this bug bugging you, or is it a non-issue? If you are affected, feel free to chime in with your experiences on the discussion boards, and be sure to let us know in the comments!