right now I'm using 2048x1546 images at 132dpi, PNG files. they look amazing but they are huge in file size.
any tips on how to get around having such large images in both dimensions and file size? (mostly file size i'm worried about here...)
The iPad screen is 1024-by-768 so to view images at 200% zoom with no pixelation you are going to need an image that's 2048 x 1546.
Find or write an image viewer that does interpolation between the pixels for the higher zoom values. The images won't be pixelated, but it will be blurred.
Increase the compression of the images. This will reduced the space they take up on the "drive", but they still need to be uncompressed for display.
Accept some pixelation at the highest zoom factors and choose a smaller image size.
Have several versions of the image for different resolutions and break the higher resolution image into chunks and only display the bits that can be seen. This is what happens with Deep Zoom type technologies as @Anders points out in his answer.
You have to balance the quality you get from the large image against the amount of memory it uses and find a compromise that's acceptable.
Do you have to use PNG files? JPGs can be compressed quite a bit more if these are photographic in nature.
I don't know how you can reduce the image dimensions but reducing the file size of PNG images is easy:
Reduce number of colors - this will have the biggest affect but might , depending on the image, also have a similar big impact on image quality (for an example open the image in your favorite image editor, run the posterize effect and save).
Drop unnecessary information and improve compression - there are a lot of programs that do that, from my experiments PNGOUT is the most effective, this will not effect the image quality.
There are a lot of other things you can do but from my experience those two are where you will get major improvements for little work.
Also, you should know those will reduce the file size, this does not mean they will reduce the amount of memory needed to display the image (in most cases they won't)