For background apps delay/latency is not a big deal. The only type of issue (that comes to mind) is when sticking with a 2G connection for a background task that needed a larger amount of bandwidth (ex. you received an email with attachments). Even then, if the email client was smart enough it could download message headers with 2G and attachments with 3G (but I don't know if you can "choose" which connection to use for what).

Anyway, it would seem like the easiest way to get started on this would be to toggle your connection based on the phones state: suspend only keeps 2G open but when you bring the phone out of suspense (turn on the screen), it goes ahead and initiates the 3G connection (if available). Go back into suspend then drop the 3G and fallback to 2G.