Why are you thinking the image has lost resolution? How did you determine this?
Are you having a problem with an AI file after printing or exporting?
Are you doing anything to the image while it is in AI, resizing, rotating?
What is the resolution of the source file?
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I experience exactly the same thing. When opening bitmaps in Illustrator CS5, they look jagged. The zoom is at 100%. The bitmaps can be anything, eg JPG with 300 ppi resolution.
I'm pulling out my hair.... It seems to print OK from Illustrator, but if you save a new PDF from Illustrator containing the bitmap, it looks exactly as in AI, bad resolution.
I never had this problem in CS2! I can't stand it how much Adobe charges for their software, but then they get simple things like this wrong....
Same for me, I have a logo as a transparent TIFF@300dpi that looks ok in AI artboard, but when I export to PDF (press quality, settings done directly by the printhouse on my macbook) it looks jagged, even at 100%. Printed with my home laser printer looks fine. I'm still trying to understand why the PDF looks awful.
I have really tried all the optiont (embed, not embed, flatten transparency, rasterize) but nothing.