-ng- was most certainly pronounced [ŋg] in Proto-Germanic, as well as in Old Norse (north), Gothic (east), and Old Saxon (west), thus occuring in all Germanic branches.
As far as I know, it also occured in Old English and Middle English. So from my limited knowledge of English, [ŋg] → [ŋ] should be a later development, though I can't give any chronological estimate.
Also, isn't there a difference between -ng and participle -ng? As in the participle suffix -ing is actually an innovation from an earlier -in (or was it -en)? The PG. suffix was *-andz which probably would have been -end or something similar in Old English.
I can't say that I have noticed it. Maybe I will now :D
Could it maybe be an influence from foreign speakers of English, who probably tend to pronounce the stop as well? My bet would be on innovation, analogy, or influence, but that's not to say I can rule out relic either. It all depends on the dialect ... but I would dare say that it's more likely to be an innovation.