this is an interesting one:
http://www.guistuff.com/css/css_imagetech1.html
so you can make a bubble-image and always cut out pieces you need for your background.
example: (not the best but the first i found *G*)
i took a nice looking bubble and divided it into 9 pieces.
4 corners, 4 edges and 1 middle-background.
now you have to know, where you are at the message/layout and choose a piece as current background.
with this technique you can make a bubble of any size without zooming in/out and getting grainy/pixelated.

EDIT: as i said... the example isn't that good because the edges aren't straight... so it would not look good if you repeatly stack them together. ^^
EDIT2: and don't ask me about the performance - i haven't got any clue. it may be loading forever on longer conversations xD