I had heard that Zetalisp had this feature, but apparently it's older than that — this was how packages originally worked. The 1979 Lisp Machine manual says:
The colon character (":") has a special meaning to the Lisp reader. When the reader sees a colon preceded by the name of a package, it will read in the next Lisp object with
packagebound to that package.
I don't know why CL degeneralized the package prefix to only work on symbols. The only reason I've heard is that a misplaced colon could accidentally cause the next form to be read in the wrong package, but that doesn't sound more dangerous than other typos like stray parentheses.
Update August 2015: It's more dangerous because unlike most typos, it has a read-time side effect: it pollutes the other package with lots of unwanted symbols.
Old Lisp manuals are a fascinating mix of futuristic and primitive. Lisp Machine Lisp also had hierarchical packages: package names were symbols, not strings, and could therefore live in other packages. But there are no earmuffs on
package, nor on any other special variables; apparently they hadn't been invented yet.