Yeah you can still decompile the game with the tools that the previous MCP versions use, and if you're familiar with parts of the code then it's possible to identify some of the files/classes/methods/etc despite the obfuscation. I doubt you'll see any mods done that way which go beyond the complexity of what Immibis did, but kudos to him and the other handful of folks who went to the trouble.
You might see some mods pop up after MCP goes up but before Forge is finished, but I don't think most users are interested in fooling with JAR mods anymore, especially in the new launcher. Forge is already being worked on thanks to the alpha release of MCP, so this may give them enough time to figure out this new networking code and everything to make their release shortly after MCP anyway.
I've played around in the alpha MCP, modifying the TNT to blow up entire villages and junk like that, and seeing what kinds of things might look different which I'd have to account for when porting mods over. But like with most other folks, I'll be waiting for Forge to make anything worthwhile.