Black Mesa Source - the long awaited Half-life fan remake - will not feature any of the Xen sections of the previous game. The developers have explained that the scenes were cut as they are being expanded.
The game will launch on September 14th but Black Mesa project leader cman2k explained on the game's official forum that it will only include content up to the Lambda Core, and added, "We believe this is a great way to provide a complete-feeling 8-10 hour experience with a solid ending, make our fans happy and help us make the best overall game possible."
The developer added that Half-Life's Xen portion will be expanded into its own game, "Development on Xen is already underway and we hope to have it finished up in a reasonable timeframe. Xen is going to basically be a complete game in its own right, it's not going to be just an epilogue to the game; more of a whole new chapter. It'll be pretty well expanded."