Why make the change?If you want to know what they say then read their blog post but I think it really comes down to cost (like most business decisions) and it's just not cost effective to develop their own any longer. Originally they made their own as there wasn't any decent open source alternative, the only two decent rendering engines out there were Internet Explorer's and Netscape's both of which were proprietary at the time.
Developing their own engines I'd guess is no longer setting them apart from the competition either any longer either as they're all pretty standards compliant and quick at rendering.
What does this mean?Well not much really, you just need to keep developing to standards and testing like normal but it does mean there's one less competitor in the market of rendering engines now.
In one way this is good in that more browsers using the same rendering engine means they'll be less 'quirks' and oddities between them and we'll also have Opera developers contributing and improving both WebKit and V8, they seem like pretty good developers too so that must be a good thing.
On the other hand it means less competition and that never seems to end well, although currently there's still Trident and Gecko out there, both of which are large and certainly make good competitors.