Stolen from:
http://www.blackberryforums.com/showthread.php?t=9514
posted by jreilley
You can set up a custom search similar to ALT I (incoming) or ALT O (Outgoing) that is tied to specific email accouts or specific subfolders
for subfolder shortcuts:
From "messages" hit S to start a search
Folder: (change this option and select the subfolder you want)
Show: Sent and Received
Type: email
click the wheel and hit "save"
Give the search a title and select a letter that will be your shortcut
click the wheel and save
for email account:
From "messages" hit S to start a search
type your email address (for the account you want to switch to) in "Name"
Folder: all folders
Show: Sent and Received
Type: email
click the wheel and hit "save"
Give the search a title and select a letter that will be your shortcut
click wheel and save
now you can use these shortcuts to go to easily switch between email accounts and subfolders