Yes, Inventory & Simple Stock Levels DOES work with RC1 (I've tested it), with the known bug that products with text attributes break the code that prevents ordering more than you have in stock. (Text attributes were a feature added to Ubercart after the Inventory module was written, and Inventory has not been revised to take account of them.)

I do plan to release a replacement, but frankly the appearance of uc_stock cut the legs out from under the whole project. It's now going to be much harder to rework the Inventory API because uc_stock destroyed the whole architecture of API. Properly implementing an inventory solution will require uc_stock to be pulled apart, and will require the new uc_inventory to be in core along with uc_stock. Since I don't have permissions to mess with core, and I'm not party to the discussions as to what goes into core and what doesn't, I fear this is going to be a long and tedious process. Certainly it's not something that can happen before 1.0 is released.

I have mostly finished writing up a detailed plan for what I want to do. When I'm done writing I'll post it here in the forums to get feedback. If I get buy-in from the community I'll go ahead and implement it. If not, the current Inventory API works for my needs so I will stick to making some small bug fixes.