Donald Francis Tovey writes about the passage in his Essays in musical analysis: "[W]e are waiting on the threshold of the original key in breathless anticipation. At last the suspense becomes too much for one of the horns, who, while the echoes of the dominant chord are still whispering, softly gives out the tonic chord of the theme. The orchestra instantly awakens and settles down to recapitulate the opening." (p. 31). He goes on to say that some famous conductors actually corrected the passage, presumably omitting or delaying the horn call until the obvious arrival of the tonic.
It is truly a strange passage. As I mentioned below, I played Beethoven's fifth symphony this past weekend, and in it, Beethoven uses a remarkably similar technique to a similar effect. The first instance occurs in the transition from the third movement into the fourth movement and is reproduced below in a reduced version:
This passage is in some ways even more extreme than the Eroica passage above. The cellos and double basses have a very long dominant pedal; the timpani has a very long tonic pedal. The first violins noodle all over this, ultimately ending up on a major subdominant chord (F major) which progresses to a dominant chord while the C in the timpani persists! What I find particularly interesting about this passage is that two timpani are used in the symphony, and they stay tuned in C and G over all four movements: Beethoven had a G at his disposal--the timpani could have played what the cellos, basses, and others were playing at this point. Also, the pedal G clashes pretty badly with the subdominant chord (I know, it's a pedal note: it's supposed to clash).
When the trio reappears in the fourth movement, Beethoven uses a similar tactic to bring us to the recapitulation:
The orchestra has been sitting on a V7 pedal for many measures; the oboe, flute, and bassoon are outlining the V7 chord melodically. The timpani enters four measures before the recapitulation with the tonic pitch (again, the only other pitch available to the timpanist is G, the dominant). The cellos and basses enter with the tonic pitch two measures before the recap.
I think Tovey's observation above describes these passages too: we're waiting breathlessly for the return to the tonic, and some of us are simply impatient.
Scott Burnham's book Beethoven Hero points to many related moments in Beethoven's music and offers highly nuanced analyses of these moments.
P.S. to undergraduate harmony students: If you write a tonic chord and dominant chord simultaneously, I will mark it wrong. :)