I know this answer doesn't apply to all of you, but for my network it works this way:

HSDPA (3.5G) = High speed downlink packet access = If you're not downloading anything, it will show 3G in the corner.

When the download starts, it will change to 3.5G, it doesn't mean that it's changing the network, it just indicates that you're transferring stuff using the HSDPA method.

Thus, if for some odd reason you just can't transfer anything, it will just show 3G and never pop up 3.5G