But is a whole nother issue, but I've had a chance to dig into the question of words such as concerning that were nicely established as participles until some well meaning but misguided prescriptivists decided to try to give them extreme makeovers. A glance at the AHD usage note on the word participle might be useful at this time. The note is at http://www.bartleby.com/61/74/P0087400.html, but I'll just quote this brief portion of it:

A number of expressions originally derived from participles have become prepositions, and these may be used to introduce phrases that are not associated with the immediately adjacent noun phrase. Such expressions include concerning, considering, failing, granting, judging by, and speaking of. Thus one may write without fear of criticism Speaking of politics, the elections have been postponed or Considering the hour, it is surprising that he arrived at all.

I have often said that it is easy to be a prescriptivist; just memorize a few rules and weep and wail and gnash teeth when presented with usages that don't follow them. Here we have a brave attempt by prescriptivists to actually describe the language, but their attempts fall short. They should leave the describing to the descriptivists. Realizing that the problem of the so-called dangling participles isn't going away they have picked a few instances of usage that may have become particularly well established and tried to cram them into some semblance of existing rules. The root of their problem is in such sentences as Turning the corner, the view was quite different. The prescriptivists call Turning the corner a dangling participle because the sentence has no noun or pronoun for the participle to modify. This is no different in grammar from the examples cited above with Speaking of politics and Considering the hour. Recasting the participles as prepositions is not the answer. The answer is recognizing the grammatical category known in other languages, notably Japanese, as the topic of the sentence. The topic may be identical to the subject but need not be. In the case of sentences with so-called dangling participles they are not identical.

Regarding but, I'll withdraw my objections. A little research has shown that but has been both a conjunction and a prepostion since OE.