I did hear on other forums - they do check your usage monitor and possibly flag your account but that was by using tether - there is a 4meg limit or less on download file I had like R30 airtime the first day I used BIS now I have nothing and guess what I can download files of 60 meg with an application.
Yes, its 5mb. It used to be much higher, but they reduced the file size limit. A year ago I was downloading 30mb mp3s and up to 50mb exe files but all that came to an end. The only files one can download now that are over 5mb as far as I know are 3gp files. I downloaded a 15MB 3gp the other day. There is NO WAY to use BIS when tethering your blackberry. You will always be charged.
RIM sets the download limits per file, in our case around 5mb's, software developers like vuclip and podtrapper bypass this by "cutting" the data into smaller chunks and splicing again on your device, in order to bypass BIS-B limitations!! Make sense??