I'd bet the culprit is the weather widget. It's quite a pig. An easier way to see what's taking things up is to use conkey, or open a terminal and use top to see whats eating memory. (You can hit 'm' in top to show the top memory eaters vs cpu users.)

That should give you a better idea of what's causing it and maybe even give you a way to fix it. For example, I found a big pig for me was the browser, in particular after using flash. It's safe to kill that, as it will re-spawn when called on, and killing it often brings things back to a snappy performance for me.

My device routinely is up for weeks at a time, only rebooting when I need to due to updates or because the battery goes dead (or a dreaded sgx spin-lock, which is happening less often as I use the phone less for navigating.) Right now it's been up for 7 days and change and is running just fine.