I recently saw a flyer that quoted from R' Chaim Kanievsky, Shlita as saying that if one find a smartphone that is not "kosher" there is no mitzvah obligation to return it. Is this in fact the halacha; meaning if one finds an object that is "assur" or would cause the owner to sin (pornography) does the finder have an obligation to return it to the owner and judge the owner favorably that he has some sort of heter to own it? Or, would this be considered placing a stumbling block before a blind person?
Hashavat Aveida, and indeed all mitzvot that are between man and his fellow man, is only applicable where the subject would benefit.
In a case where returning an object would harm the person more than benefiting him, there is no mitzva. Perhaps this is the rationale behind R. Chaim Kanievsky's ruling.
For example, if storage costs charged to the owner would outweigh the item's value, the finder should sell the item and hold the money until it can be returned. Another example (no source for this but using common sense), there would be no mitzva to return a bottle of liquor to an alcoholic, or a pack of cigarettes to a smoker, certainly if they were trying to kick the habit.
The Rambam (Aveidah 11:13, also see Choshen Mishpat 263:1) writes:
"The following rule applies when a person finds a sack or a large basket. If he is a sage or a respected elder, who would not usually carry such articles himself, he is not obligated to concern himself with them.
"He should judge his status in the following way. If the article were his own and he would return it, so too, is he obligated to return an article belonging to a colleague. If, however, he would not forgo his honor even if the article were his own, he is not obligated to return a similar article belonging to a colleague."
So if one believes it is embarrassing to carry an iPhone, then one would not be required to return such an item.
In Choshen Mishpat 65:8, the Sefer Me'irat Enayim writes that if one finds a shtar in the street which includes interest, one does not return it, rather one rips it up. He quotes the Teshuvos Maimoniyos Mishpatim 59 that even if it means a loss to the person, still the shtar should be ripped up. So here we see a case of a "davar issur" and not only does one not return it, but he should destroy it.
Also, in the sefer Mishpat Ha-Aveidah (259, p24, in the footnotes, http://hebrewbooks.org/pdfpager.aspx?req=20420&st=&pgnum=37&hilite=) he considers a case of finding immodest clothing (which should be sold to non-Jews and the money returned to the Jew), forbidden images of constellations (which should be broken first then the broken pieces returned), and a forbidden children's doll (which can be returned whole since some authorities permit it anyway).