Here's my experience with MicroSD cards.
Blackberry 8120 Smartphone (EDGE, Wi-Fi) v.18.104.22.168 (Platform 22.214.171.124)
Cryptographic Kernel v.126.96.36.199c
MicroEdition Configuration CLDC-1.1
MicroEdition Profile: MIDP-2.0
MicroEdition JTWI Version: 1.0
MicroEdition Media Version: 1.1
MicroEdition PIM Version: 1.0
MicroEdition File Version: 1.0
MicroEdition Bluetooth Version: 1.1
Micro Edition Location Version: 1.1
MicroEdition Security and Trust Services (APDU) Version: 1.0
WLAN Version: 188.8.131.52
Sandisk 8GB Micro-SDHC card from Low Price Memory (£17.44 inc. delivery). Insert card in phone, format card, connect phone to PC, select 'Mass Storage On'. After a short delay phone appears in Windows Media Player 11 as having 7.6GB free. I've shifted about 3GB worth of albums across so far, but with no problems. The only thing I found was that the first time you go into 'Media' - 'Music' after shifting music across, the phone takes awhile to find all the information on the card, but once it has they appear right away the next time you go in.