Google reports about 37,000 hits for "shameless plug of/for my" or "shamelessly plugging my," and the total would be a lot bigger if you allowed for variants like "my shameless self-promotion" and so forth. The phrases are much more common now than they were in the old media, mostly because the new media have dramatically increased the opportunities for self-exposure — Google Blogsearch alone turns up more than 4000 hits for the phrases. The vast majority of these are associated with creative works and activities, in a broad sense of the term: people apologize for shamelessly touting their books, TV appearances, t-shirt designs, videos, high-school band performances, blogs, and new CD's. (Others apologize for shamelessly plugging their hairdresser or a PC they have for sale on Ebay, which seems to me a little unclear on the concept — what's to be ashamed of?) 
The modifier accomplishes several things at once: it concedes that the self-promotion is an impropriety, but one venial enough to be joked about; and it averts the reader's censure with preemptive self-reproach. It reminds me of the way H. W. Fowler described the use of apologies like "saving the reader's reverence" and "if we may adopt the current slang": "A refinement on the institution of the whipping boy, by which [writers] not only have the boy, but do the whipping."
My new collection The Years of Talking Dangerously was published today by PublicAffairs.