Gmail is trying to help combat the spread of viruses by implementing a "feature" that prevents emails containing attachments of certain file types from being delivered to Gmail accounts. With some simple workarounds, you can get your files delivered to Gmail users while still helping prevent the spread of viruses...
One way that Gmail is helping to prevent the spread of viruses is to prevent the delivery of messages sent from outside of Gmail that .exe and .vbs (and possibly other) extensions. These file attachments are very easy to inadvertently execute, and are a common source of viruses. Gmail is trying to make it at lease a bit more difficult to inadvertently execute these types of files.
If you are sending an email to a Gmail account that contains either extension of .exe or a .vbs (and possibly others) the message WILL bounce as undeliverable. The message will also bounce if these file types are contained within a .ZIP file. If you are sending an email from an outside account to a Gmail user, simply changing the extention to something else appending something like ".txt" lets the message pass through without issue. You could also use another archive format like WinAce or WinRAR as they are not filtered (yet). The point is that the file won't be immediatly and inadvertently executable without some recipient intervention.
Again, note that this only affects inbound email originating from another email provider sent to a Gmail account. This does not affect Gmail users sending email to another Gmail user or to an email address outside of Gmail. Obviously, this isn't a perfect solution, but it's a good start.