As far as I can say from the trac changes in the WordPress Trac, the last Revision affecting the default theme before 2.5.1 was Rev 7553. Version 2.5 was tagged in Rev #7567. If I am right (not that familiar with SVN repositories, I just know a few basics), the default themes in 2.5 and 2.5.1 are identical.
I hope, this answers your question.
You should consider to make all your code PHP5 compatible and perhaps ask your host to upgrade, too, because PHP4 support has been dropped since January 2008, I think.
I am not quite sure, if WordPress still supports PHP4. All my servers are already running PHP5, so I do not experience any problems.
