@Jeff, Just jailbreak your device with the firmware you have. Before you go ahead and jailbreak, simply just backup your device with official iOS firmware and data, etc. As soon as you backed up, you are free to jailbreak.
Once iOS 5 comes out, simply just save your SHSH blob for backing up your jailbroken firmware and data. I would link a tutorial, but I do not know the firmware you will be jailbreaking or the device (I assume iPhone 4, iPhone 3GS, iPod Touch 3/4, or iPad 1/2), so I suggest googling that later on. As soon as your SHSH blob is kept, go ahead and restore to official iOS firmware (the one you currently had before you jailbreak) - (just simply connect your device to iTunes and click restore).
After that, go ahead and click update (to update to iOS 5) and you will begin downloading the firmware. After it finishes downloading, it will update your iDevice. After everything is done installing, make sure you set up your device and make sure you then restore from a backup (the one you made before you jailbreaked) and restore that to your device.
NOTE: If you have a lot of data changing with your iDevice (example an iPhone), then I would suggest not jailbreaking and using an official firmware and backing up then, then updating to iOS 5, then restore backup, so all data is current as possible with iOS 5.
IF NOT, then go ahead and attempt to follow my lengthy process above the NOTE.