Ok, there are some more details about T-Mo plans in »[T-Mobile] PAYG and PBTD prepaid plans for backup purposes
. Summary: PAYG expiration date resets when you do a refill, instead of the days adding up. However, once you reach Gold status (either through several refills to a 100$ total, or through a single 100$ refill), all subsequent refills reset the expiration date to a year from any refill, instead of just 90 days. None of these rules appear in fineprint, it seems, but they do appear in official faqs (see the tmo thread above for more details).
It's still unclear / unconfirmed what kind of expiration rules are on the PbtD plans.
Also, I thought a little bit more about the 5GB@4G plan, and some common engineering sense (which almost always conflicts with marketing) dictates that using those 5GB over 3G makes use of less resources than 5GB over 2G. I talked to my vendor, and I plan on having about 10MB/day through GPRS; that's 300MB/month on GPRS. Logically, it seems several times more than 5GB@HSPA+; is it at all likely that they will disable an account that's using 300MB/month through GPRS? How swamped are 2G / 2.5G networks nowadays? The whole 2G GSM spectrum is shared between all kinds of 2G / 2.5G and 2.75G devices, right? How efficient or inefficient is periodic (every 10 to 60 seconds) short message passing through TCP or UDP over GPRS compared to the same messages being passed over SMS?