Update: GMail now offers IMAP support, a good alternative if supported by your client…
While on the road I download mail from GMail to my PDA (an N800) via POP. However, when I subsequently checked the same GMail account from my laptop (via POP) it would skip over the previously downloaded email, leaving me with emails in two places.
Today, I discovered if you change your GMail login name in both your PDA client and desktop mail applications to
recent:<username>@gmail.com then GMail will download all incoming email to both. The only side effect is it seems to download emails that you sent to others via the GMail web UI but I can live with that as I don’t use the web UI often.
NOTE: A word of caution – before you change your login name in your desktop mail application make sure you empty/archive your inbox first as it (Apple Mail in my case) will re-download your last 30days worth of email again leaving you with duplicates to delete. It behaves nicely from then on though.
See the GMail Help Center for more.