I think the page guids are preserved, you could try to save that instead of the ID. I could be wrong though.
If possible, use properties (of type Page) to keep track of pages rather than saving references to pages externally, e.g. in config files. That usually causes less headaches.
Mangus is correct that the guid based identifier for the page is preserved during mirroring (unless you mirror within the same site or another site in an enterprise setup).
The integer based identifiers are though not preserved, the reason is that it would be hard to preserve the id, e.g. there might already exist a different page with that id in the destination site.
We want to mirror the database from one environment to another. Can we keep the page id's same when we do the mirroring in all th environments? The problem is if we have set some page id's in one env when we mirror the page ids are changing.