This matter has already been discussed in the B-Greek forum; in addition to those already said, I should add that it seems to me that the sentence,
Ἐγένετο δὲ ἐν τῷ βαπτισθῆναι ἅπαντα τὸν λαὸν καὶ Ἰησοῦ βαπτισθέντος καὶ προσευχομένου ἀνεῳχθῆναι τὸν οὐρανὸν καὶ καταβῆναι τὸ πνεῦμα τὸ ἅγιον σωματικῷ εἴδει ὡς περιστερὰν ἐπ’ αὐτόν...
is in fact made up of two sentences interweaved together:
1. Ἐγένετο δὲ ἐν τῷ βαπτισθῆναι ἅπαντα τὸν λαὸν (...) ἀνεῳχθῆναι τὸν οὐρανὸν καὶ καταβῆναι τὸ πνεῦμα τὸ ἅγιον σωματικῷ ...
2. καὶ Ἰησοῦ βαπτισθέντος καὶ προσευχομένου (...) εἴδει ὡς περιστερὰν ἐπ’ αὐτόν ...
I think this is a very beautiful way to encompass a large picture - vivid indeed and dynamic - in relatively few words.
