Very good points made there Andy, although I can see one point of concern for songwriters/ composers posting their work on the internet before it's published. If there are any unscrupulous companies out there scouring the net for a good tune, what's to stop them from using yours and registering it to themselves? It's not other writers that I'm worried about - it's companies/ rip off merchants.
I do agree, though, the risk is very small indeed.
But that's just it. If you've uploaded your mp3s onto a site such as Myspace, the exact date and time is recorded in the site's log.
Whatever someone did to try and "register" the song themselves it would have to be after the date you uploaded it.
Remember, the copyright of a song is automatically owned by the composer/s. There is no formal method of registration that can override that.
If a company wants to make money out of your song, the most sensible way for them to do so is to contact you and negotiate some kind of deal with you. Trying to steal it could well cost them more money than it's worth, not to mention their professional reputation. Plus no court in the land is going to believe a company composed a song all by themselves.
Intellectual property theft is not impossible by any means, but I'd be more suspicious of the people you co-write with etc. than anyone else. I remember a number of years ago Sammy Hagar wrote a number of tracks for an (at the time) more established artist, some of which ended up being credited as co-writes between Hagar and said artist and some as I recall weren't even credited to Hagar at all.
I've also heard rumours that 'Love Is All Around' wasn't actually written by Reg Presley, but then there's rumours that Elvis is still alive, so I'm not sure what to believe.