I am wondering if I upgrade from Ubuntu 9.04 to Ubuntu 9.10 on my laptop (a dell inspiron 1545) using broadcom wireless drivers; will my wireless break?
When I side along installed Ubuntu 9.10 last time, the wireless didn't work.
Given that your wireless didn't work on Ubuntu 9.10 out of the box, I expect you'll have similar trouble upgrading 9.04 -> 9.10.
Linux drivers are tied to the kernel version in use, and the upgrade to 9.10 will install a new kernel. You might be able to keep using the older kernel (and thus the older working wireless driver), but that might cause problems with other aspects of 9.10.
I'd recommend working on your along-side installation to figure out how to get your driver working. Then when you do upgrade the 9.04 installation and the wireless breaks, you'll know what to do to get it working again.
It depends if you used the broadcomm wireless drivers or just downloaded the firmware.
If you downloaded the firmware (either with fwcutter or are using openfwwf), then you should be fine.
If you are using the binary-only driver, you're going to encounter similar issues.
My recommendation, as hard as it might be: find a nice wireless card with an ath5k or ath9k chipset. Those drivers are in-tree and completely Free Software.