It's only Pages where this is an issue. Posts are fine.
My site does that same behavior. Not matter what number I pick, say 750, WordPress appends "-2" (unless I have a page with that slug already, in which case it's "-3"). If I go to example.com/750/, WordPress does redirect me to the correct example.com/750-2/, in other words, there is no 750 slug.
Perhaps it's a plugin altering the behavior or some strange overlooked server setting. I've looked for a page, post, attachment or whatever with the slug and haven't found any (the site is an almost fresh test install with only health check plugin enabled.)
I have been able to reproduce Hakre's experience by using posts. (I think he was publishing posts because posts don't seem have this issue, only pages.) I haven't experimented with custom post types as it applies to this.
Seems like a bug to me.
RegardlessSeems like a bug to me, but regardless of whether or not it's a bug, you can not use numeric slugs on pages.
This is because number slugs like "/750/" interfere with posts years "/2010/" etc. and WordPress can't tell the difference.
If you save two pages to draft with the same number (say 750) then WordPress assigns the slug 750-2 to both. Upon publish, the first will stay 750-2 while the second will update to 750-3. There is no normal way to get slug 750 on a page as of 3.0
Update: bug at http://core.trac.wordpress.org/ticket/14238http://core.trac.wordpress.org/ticket/11917 is the ticket that outlawed numeric slugs.