That would be so stupid. Really. Celtic are a much bigger team than Newcastle. English people who think otherwise are wrong, sorry. With Celtic he may not be winning Champions Leagues, but with Newcastle he won't be playing in them at all. With Celtic he may be playing against the likes of Inverness and Falkirk, but that has to be a comfort level to him. With "pushing on" to the Premiership, there's clearly a big positive and a big negative possibility. Stilian Petrov, although not playing too terrible lately, is a case of being told to "go to the Premiership!" all the time [particularly by Stoichkov], and when he did, he flopped, had an awful first season, and far from spectecular the next two. The only reason O'Neill kept him was because he likes him. Since he left Celtic, he's played zero seconds of Champions League football. Maybe Villa will get in this year, and by that stage it'll be his 4th season with them, and he'll not be good enough to make a decent impact in the Champions League [in my opinion].
If Boruc was offered a regular starting place by a top 4 team, although I wouldn't wanna sell, I wouldn't blame him too much for accepting it.