OK this has worked on a couple computers, so I'm pretty sure this'll be safe to do:
1. goto
http://www.4shared.com/file/kjJSrQ2B/minecraft-15.html
2. download the minecraft-1-5.jar (or whatever its called)
3. click start-run, then type in %APPDATA%\.minecraft\bin
4. when you run minecraft and it updates, it deletes the old minecraft.jar and replaces it with the newest one (1.65 or so right now).
5. rename minecraft.jar to minecraft.old (or whatever)
6. copy the file you downloaded in step 2 into this folder, rename it minecraft.jar
7. run minecraft - it thinks you've already updated, so won't do it again, you should now be running 1.5 and be able to connect to the 2+2 server
note: the minecraft.jar that you downloaded will be overwritten the next time they put out a new version of Minecraft (it will autoupdate), so you'll have to do these steps again until they upgrade the server.
If you run into problems, post here and I'll try to check in regularly.