(This is from pre-blog, as is evident from the lack of Hebrew or links.)
In parashat Tetzaveh, there is a curious pasuk, regarding the Tzitz: (Shemot 28:38) And it shall be upon Aaron's forehead, that Aaron may bear the iniquity of the holy things, which the children of Israel shall hallow in all their holy gifts; and it shall be always upon his forehead, that they may be accepted before the LORD.
The curious phrase is "that Aaron may bear the iniquity of the holy things," ViNasa Aharon Et-Avon HaKodoshim. The Kodoshim typically refers to Korbanot.
The trouble is, what iniquity are we talking about in regard to the korbanot? I will offer three traditional answers, and then offer some completely unsubstatiated and wild speculation.
First, Rashi explains that the verb "bear," Nasa, means pardon (slicha), and yet it still retains a different connotation, that Aharon carries the burden of iniquity; thus the iniquity is removed from the korbanot. To identify the "iniquity," Rashi cites the gemara in Menachot 25a to say that it refers to fats and blood offered while they are Tamei. The Mishna there (top of 25a) states that if the Kometz (of the mincha) becomes Tamei, the Tzitz makes it accepted, but if it is Yatza (leaves proper areas), the Tzitz does not make it accepted (meratze), for the Tzitz only makes Tamei acceptable, but not Yotzei acceptable. The gemara there shows how this is derived from this pasuk. Basically, the pasuk says that the Tzitz is Nasa the iniquity of the kodoshim, such that "that they may be accepted before the LORD." And the iniquity (avon) belongs to the kodoshim and not to those who are bringing it.
Targum Yonatan ben Uziel explains the Avon Kodoshim as the iniquity of promising to bring a korban and then not doing it. Thus, the Avon of the Jews who are false in their promises of korbanot, that iniquity Aharon bears for them and makes those sins be forgiven. Tg Yonatan says this not in translating the phrase "the iniquity of the holy things," but rather, he translates later in the pasuk "which the children of Israel shall hallow in all their holy gifts" as "which the children of Israel shall hallow in all their holy gifts that they are false in them (DiMishakrin Behon)."
Rashbam says that according to the pshat it is not like Rashi, referring to the Tumah of the kodoshim, but rather, the korbanot that the Jews bring as Olah, Chatat, or Asham offerings - the tzitz together with the korban helps to recall them for good before Hashem and for a recollection on behalf of the Jews that their sins should be forgiven them.
Finally, some random, unsubstantiated suggestions. Look at the context.
25:34) A golden bell and a pomegranate, a golden bell and a pomegranate, upon the hem of the robe round about. 35) And it shall be upon Aaron to minister: and his sound shall be heard when he goeth in unto the holy place before the LORD, and when he cometh out, that he die not.
Then, 36) And thou shalt make a plate of pure gold, and grave upon it, like the engravings of a signet, HOLINESS TO THE LORD. 37) And thou shalt put it on a blue lace, that it may be upon the mitre; upon the forefront of the mitre it shall be. 38) And it shall be upon Aaron's forehead, that Aaron may bear the iniquity of the holy things, which the children of Israel shall hallow in all their holy gifts; and it shall be always upon his forehead, that they may be accepted before the LORD.
In pasuk 35, what does it mean that Aharon's sound should be heard... that he die not? Rashi there says this teaches that if he is missing one of the priestly garments when he enters the Kodesh, he is chayav mita in the hands of Heaven. Tg Yonatan seems to imply some kind of immediate death, translating, and he will not dies in Esha Metzalhava (some type of fire).
I would suggest that the ringing of the bells announcing his entering and departure from the Kodesh somehow prevents some harm befalling him. Perhaps this is averting some punishment, or perhaps this announcing causes some change in the Kodesh such that it will receive him? I think it is the former - only one who is properly announced can emerge from the Kodesh unscathed.
In view of this, perhaps a similar idea is present in the tzitz. Pasuk 36 says that Aharon was to wear a forehead plate (tzitz) on which was written Kodesh LaHashem, Holy to the Lord, or (in the translation I took,) Holiness to the Lord. He had to wear this tzitz at all times while doing the service. Perhaps it was similarly to identify him as the Holy One to Hashem, designated to perform this service. After all, it is presumption to touch and deal with korbanot, which go to Hashem, and only the Kodesh LaHashem could deal with them. Thus, the tzitz helped Aharon bear the Avon HaKodoshim, the "iniquity" present in any human dealing with things going to Hashem, because he was designated.