Get the 5D. Or, even better, get an SLR with an APS-sized sensor (10D, 20D, Rebels) so that you get more out of your long lenses. The full-frame 5D will need a 400mm to get the kind of zoom you get on a Panasonic FZ, but the smaller-sensor SLRs will only need a 300mm lens, so you can save some money there or get a faster or better lens.
The smaller-sensor cameras may be a tad more noisy than the 5D but they are still MUCH sharper than the FZs, especially if you use a nice lens (which you will have money to get, if you save $2000 by getting an APS-sensor SLR instead of the 5D).
I was recently in the same position - wanting to go from a Panazonic FZ10 to an SLR. I loved the big, fast, image-stabilized zoom in the Panasonic, and I knew that I would eventually be spending a good deal of money to buy lenses for my SLR that could approach that kind of performance. If you have a budget of $3000 and you want to get nice zoom shots, I'd recommend spending around (or less than) $1000 on the body and around $2000 on the lenses, rather than the other way around. For example, I'd recommend the Canon 100-400L-IS lens to get the kind of stabilized zoom you get with the Panasonic. (The lens is a tad slower, but you can use higher ISOs than with the Panasonic. My FZ is always set at 50, but my 10D is usually at 100, maybe 200 if it's not too bright out).
I do a lot of aviation photography, and go to about 10 airshows a year. Last year I went to Nellis with my FZ10, and this year I had the 10D with the 100-400L on it. I happened to take a shot that was very similar to one I took last year, so you can REALLY see the vast improvement that even a low-end SLR can offer relative to a Panasonic FZ, especially when a nice lens is used:
http://www.airshowfan.com/favorite-plane-pix/51.jpg
http://airshowfan.com/nellis05/20.JPG
And while the one taken with the FZ can't really be cropped too much or at all, the one taken with the 10D can be cropped and enlarged, since it's so much sharper.
So as lovely as the Panasonics are (I still have my FZ10 and use it regularly), they won't match the sharpness that an SLR offers, especially if you buy a nice lens with all this money you want to spend on a 5D.