Delivering messages to offline resources too

Situation: I use two resources (say, "notebook" and "pda") with equal priorities, so when both resources are online, messages are delivered to both devices. Notebook is always online, pda is online when i'm out. So, i'm always get latest messages, great.
Problem: sometimes my pda is disconnected (for example, metro or poor coverage) and messages (in the middle of conversation) are delivered to notebook. I want to get missed messages on pda.
Limitation: notebook must always stay online, because i want to see all history (for example to copy-paste data easily) when i get back home.
Question: it it possible? How? Maybe some server-side history sync module? Thanks.